f2fs: fix to avoid panic in dec_valid_node_count()

f2fs: fix to avoid panic in dec_valid_node_count()

As Jungyeon reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=203213

- Overview
When mounting the attached crafted image and running program, I got this error.
Additionally, it hangs on sync after running the this script.

The image is intentionally fuzzed from a normal f2fs image for testing and I enabled option CONFIG_F2FS_CHECK_FS on.

- Reproduces
mkdir test
mount -t f2fs tmp.img test
cp a.out test
cd test
sudo ./a.out
sync

 kernel BUG at fs/f2fs/f2fs.h:2012!
 RIP: 0010:truncate_node+0x2c9/0x2e0
 Call Trace:
  f2fs_truncate_xattr_node+0xa1/0x130
  f2fs_remove_inode_page+0x82/0x2d0
  f2fs_evict_inode+0x2a3/0x3a0
  evict+0xba/0x180
  __dentry_kill+0xbe/0x160
  dentry_kill+0x46/0x180
  dput+0xbb/0x100
  do_renameat2+0x3c9/0x550
  __x64_sys_rename+0x17/0x20
  do_syscall_64+0x43/0xf0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

The reason is dec_valid_node_count() will trigger kernel panic due to
inconsistent count in between inode.i_blocks and actual block.

To avoid panic, let's just print debug message and set SBI_NEED_FSCK to
give a hint to fsck for latter repairing.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: fix build warning and add unlikely]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>