f2fs: fix to do sanity check on inline_dots inode

f2fs: fix to do sanity check on inline_dots inode

As Wenqing reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=215765

It will cause a kernel panic with steps:
- mkdir mnt
- mount tmp40.img mnt
- ls mnt

folio_mark_dirty+0x33/0x50
f2fs_add_regular_entry+0x541/0xad0 [f2fs]
f2fs_add_dentry+0x6c/0xb0 [f2fs]
f2fs_do_add_link+0x182/0x230 [f2fs]
__recover_dot_dentries+0x2d6/0x470 [f2fs]
f2fs_lookup+0x5af/0x6a0 [f2fs]
__lookup_slow+0xac/0x200
lookup_slow+0x45/0x70
walk_component+0x16c/0x250
path_lookupat+0x8b/0x1f0
filename_lookup+0xef/0x250
user_path_at_empty+0x46/0x70
vfs_statx+0x98/0x190
__do_sys_newlstat+0x41/0x90
__x64_sys_newlstat+0x1a/0x30
do_syscall_64+0x37/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xae

The root cause is for special file: e.g. character, block, fifo or
socket file, f2fs doesn't assign address space operations pointer array
for mapping->a_ops field, so, in a fuzzed image, if inline_dots flag was
tagged in special file, during lookup(), when f2fs runs into
__recover_dot_dentries(), it will cause NULL pointer access once
f2fs_add_regular_entry() calls a_ops->set_dirty_page().

Fixes: 510022a85839 ("f2fs: add F2FS_INLINE_DOTS to recover missing dot dentries")
Reported-by: Wenqing Liu <wenqingliu0120@gmail.com>
Signed-off-by: Chao Yu <chao.yu@oppo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>