sysfs: replace WARN() with pr_debug when sysfs_remove_group() failed

sysfs: replace WARN() with pr_debug when sysfs_remove_group() failed

Orabug: 26374902

There is no enough error handling in block device adding/registration
path, for example,

device_add_disk()
  blk_register_queue()

When kernel returns from device_add_disk(), no return value to tell
us it was successful or not --- that suggests it would always succeed,
and according to this assumption, then during block device removal/
unregistration steps,

sd_remove()
  del_gendisk()
    blk_unregister_queue()

dpm_sysfs_remove(), blk_trace_remove_sysfs() will be called blindly,
though there is likely no 'trace' 'power' sysfs groups there because
actually blk_register_queue()/device_add() failed somewhere. thus
causes WARN flood emitted from sysfs_remove_group() as following triggered
by unloading fnic driver:

 modprobe -rv fnic

[  122.081398] WARNING: CPU: 14 PID: 11709 at fs/sysfs/group.c:224
        sysfs_remove_group+0x9c/0xa0()
[  122.081399] sysfs group 'trace' not found for kobject 'sdb'
[  122.081424] CPU: 14 PID: 11709 Comm: modprobe Tainted: G        W
        4.1.12.x86_64 #2
[  122.081425] Hardware name: Cisco Systems Inc UCSBXXxx
[  122.081425]  0000000000000286 00000000d03792ff ffff881037823ad8
        ffffffff8173605d
[  122.081427]  ffff881037823b30 ffffffff81a2b9bc ffff881037823b18
        ffffffff810862aa
[  122.081428]  ffff88103974a000 0000000000000000 ffffffff81ba4080
        ffff882037d45080
[  122.081430] Call Trace:
[  122.081432]  [<ffffffff8173605d>] dump_stack+0x63/0x81
[  122.081434]  [<ffffffff810862aa>] warn_slowpath_common+0x8a/0xc0
[  122.081435]  [<ffffffff81086335>] warn_slowpath_fmt+0x55/0x70
[  122.081437]  [<ffffffff8129321c>] ? kernfs_find_and_get_ns+0x4c/0x60
[  122.081439]  [<ffffffff81296b5c>] sysfs_remove_group+0x9c/0xa0
[  122.081441]  [<ffffffff811675a4>] blk_trace_remove_sysfs+0x14/0x20
[  122.081444]  [<ffffffff81312605>] blk_unregister_queue+0x65/0x90
[  122.081446]  [<ffffffff81320f26>] del_gendisk+0x126/0x290
[  122.081449]  [<ffffffffa0091281>] sd_remove+0x61/0xc0 [sd_mod]
[  122.081452]  [<ffffffff81492fb7>] __device_release_driver+0x87/0x120
[  122.081454]  [<ffffffff81493073>] device_release_driver+0x23/0x30
[  122.081456]  [<ffffffff814928f8>] bus_remove_device+0x108/0x180
[  122.081457]  [<ffffffff8148eca0>] device_del+0x160/0x2a0
[  122.081459]  [<ffffffff814d8feb>] __scsi_remove_device+0xcb/0xd0
[  122.081461]  [<ffffffff814d7524>] scsi_forget_host+0x64/0x70
[  122.081462]  [<ffffffff814cac0b>] scsi_remove_host+0x7b/0x130
[  122.081466]  [<ffffffffa016fc47>] fnic_remove+0x1b7/0x4a0 [fnic]
[  122.081469]  [<ffffffff8138434f>] pci_device_remove+0x3f/0xc0
[  122.081472]  [<ffffffff81492fb7>] __device_release_driver+0x87/0x120
[  122.081474]  [<ffffffff81493a38>] driver_detach+0xc8/0xd0
[  122.081478]  [<ffffffff81492c19>] bus_remove_driver+0x59/0xe0
[  122.081479]  [<ffffffff814942e0>] driver_unregister+0x30/0x70
[  122.081482]  [<ffffffff81382dba>] pci_unregister_driver+0x2a/0x80
[  122.081486]  [<ffffffffa01808cc>] fnic_cleanup_module+0x10/0x7a [fnic]
[  122.081488]  [<ffffffff8110e8ec>] SyS_delete_module+0x1ac/0x230
[  122.081490]  [<ffffffff81028666>] ? syscall_trace_leave+0xc6/0x150
[  122.081491]  [<ffffffff8173dcee>] system_call_fastpath+0x12/0x71
[  122.081502] ---[ end trace 29ba5813719045a4 ]---

WARNING: CPU: 14 PID: 11709 at fs/sysfs/group.c:224
        sysfs_remove_group+0x9c/0xa0()
[  122.095724] sysfs group 'power' not found for kobject 'target2:0:4'
[  122.095790] CPU: 14 PID: 11709 Comm: modprobe Tainted: G        W
        4.1.12.x86_64 #2
[  122.095793] Hardware name: Cisco Systems Inc UCSBXXxx
[  122.095795]  0000000000000286 00000000d03792ff ffff881037823af8
        ffffffff8173605d
[  122.095800]  ffff881037823b50 ffffffff81a2b9bc ffff881037823b38
        ffffffff810862aa
[  122.095803]  ffff88103782
[  122.095807] Call Trace:
[  122.095814]  [<ffffffff8173605d>] dump_stack+0x63/0x81
[  122.095818]  [<ffffffff810862aa>] warn_slowpath_common+0x8a/0xc0
[  122.095822]  [<ffffffff81086335>] warn_slowpath_fmt+0x55/0x70
[  122.095827]  [<ffffffff8129321c>] ? kernfs_find_and_get_ns+0x4c/0x60
[  122.095831]  [<ffffffff81296b5c>] sysfs_remove_group+0x9c/0xa0
[  122.095839]  [<ffffffff8149b7e7>] dpm_sysfs_remove+0x57/0x60
[  122.095843]  [<ffffffff8148ebc6>] device_del+0x86/0x2a0
[  122.095847]  [<ffffffff8148e1f9>] ? device_remove_file+0x19/0x20
[  122.095854]  [<ffffffff814983ae>] attribute_container_class_device_del
        +0x1e/0x30
[  122.095858]  [<ffffffff814985c2>] transport_remove_classdev+0x52/0x60
[  122.095862]  [<ffffffff81498570>] ? transport_add_class_device+0x40/0x40
[  122.095866]  [<ffffffff81497f1c>] attribute_container_device_trigger
        +0xdc/0xf0
[  122.095870]  [<ffffffff81498525>] transport_remove_device+0x15/0x20
[  122.095875]  [<ffffffff814d4df5>] scsi_target_reap_ref_release+0x25/0x40
[  122.095879]  [<ffffffff814d68fc>] scsi_target_reap+0x2c/0x30
[  122.095883]  [<ffffffff814d8fa6>] __scsi_remove_device+0x86/0xd0
[  122.095887]  [<ffffffff814d7524>] scsi_forget_host+0x64/0x70
[  122.095891]  [<ffffffff814cac0b>] scsi_remove_host+0x7b/0x130
[  122.095900]  [<ffffffffa016fc47>] fnic_remove+0x1b7/0x4a0 [fnic]
[  122.095909]  [<ffffffff8138434f>] pci_device_remove+0x3f/0xc0
[  122.095915]  [<ffffffff81492fb7>] __device_release_driver+0x87/0x120
[  122.095922]  [<ffffffff81493a38>] driver_detach+0xc8/0xd0
[  122.095930]  [<ffffffff81492c19>] bus_remove_driver+0x59/0xe0
[  122.095934]  [<ffffffff814942e0>] driver_unregister+0x30/0x70
[  122.095941]  [<ffffffff81382dba>] pci_unregister_driver+0x2a/0x80
[  122.095952]  [<ffffffffa01808cc>] fnic_cleanup_module+0x10/0x7a [fnic]
[  122.095957]  [<ffffffff8110e8ec>] SyS_delete_module+0x1ac/0x230
[  122.095961]  [<ffffffff81028666>] ? syscall_trace_leave+0xc6/0x150
[  122.095966]  [<ffffffff8173dcee>] system_call_fastpath+0x12/0x71
[  122.095968] ---[ end trace 29ba5813719045a6 ]---

While, refactoring block device code seems not valuable if just
because of above noisy but not so dangerous WARN flood.

So this patch suppress the warning flood by replacing WARN() with
pr_debug() as shortcut before refactoring all related block device
code.

This issue also could be reproduced with stable v4.12 kernel.

(Upstream maintainer Greg K-H refused to apply this "workaround / shortcut",
He insisted the issue should be fixed in block device subsystem, that means refactoring
all block device/SCSI drivers and all relevant block layer code, that is not practical task,
it is too expensive, and we couldn't wait for the upstream refactoring,
So this patch is specific to UEK4 code,
*NOTE*, there will be no WARNNING in sysfs_remove_group(), this doens't affect
other WARN_ONCE() in kenrel )

Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>
Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>