f2fs: fix to avoid panic in dec_valid_block_count()

f2fs: fix to avoid panic in dec_valid_block_count()

[ Upstream commit 5e159cd349bf3a31fb7e35c23a93308eb30f4f71 ]

As Jungyeon reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=203209

- Overview
When mounting the attached crafted image and running program, I got this error.
Additionally, it hangs on sync after 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
cc poc_01.c
./run.sh f2fs
sync

 kernel BUG at fs/f2fs/f2fs.h:1788!
 RIP: 0010:f2fs_truncate_data_blocks_range+0x342/0x350
 Call Trace:
  f2fs_truncate_blocks+0x36d/0x3c0
  f2fs_truncate+0x88/0x110
  f2fs_setattr+0x3e1/0x460
  notify_change+0x2da/0x400
  do_truncate+0x6d/0xb0
  do_sys_ftruncate+0xf1/0x160
  do_syscall_64+0x43/0xf0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

The reason is dec_valid_block_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>
Signed-off-by: Sasha Levin <sashal@kernel.org>