sparc64: fix cdev_put() use-after-free when unbinding an LDom

sparc64: fix cdev_put() use-after-free when unbinding an LDom

After turning on slub_debug=P kernel option, a kernel panic happens when
unbinding an LDom. This suggests that there is memory corruption.
The memory corruption is caused by vlds_fops_release() freeing a memory
structure containing a cdev. The cdev is needed by fs/file_table.c
after the file is released.

The common approach to solve this issue is to add a kobject member
in the structure and set it to be the parent of cdev. The kobject is
then responsible to free the structure when the reference count is
zero. The reference solution is based on the following patch.

https://patchwork.kernel.org/patch/8985881/

Orabug: 25911389

Signed-off-by: Thomas Tai <thomas.tai@oracle.com>
Reviewed-By: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>
Reviewed-by: Tom Saeger <tom.saeger@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>