fat: avoid parent link count underflow in rmdir

fat: avoid parent link count underflow in rmdir

Corrupted FAT images can leave a directory inode with an incorrect
i_nlink (e.g. 2 even though subdirectories exist). rmdir then
unconditionally calls drop_nlink(dir) and can drive i_nlink to 0,
triggering the WARN_ON in drop_nlink().

Add a sanity check in vfat_rmdir() and msdos_rmdir(): only drop the
parent link count when it is at least 3, otherwise report a filesystem
error.

Link: https://lkml.kernel.org/r/20260101111148.1437-1-zhiyuzhang999@gmail.com
Fixes: 9a53c3a783c2 ("[PATCH] r/o bind mounts: unlink: monitor i_nlink")
Signed-off-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Reported-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Closes: https://lore.kernel.org/linux-fsdevel/aVN06OKsKxZe6-Kv@casper.infradead.org/T/#t
Tested-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>