minix: reject rmdir of a directory with a corrupted link count

minix_rmdir() calls minix_unlink(), which drops the victim's link
count once, and then drops it a second time for the "." entry. An
image where an empty directory has i_nlink < 2 makes the second drop
hit drop_nlink() with i_nlink == 0:

  WARNING: fs/inode.c:408 at drop_nlink
   minix_rmdir+0xe9/0x150
   vfs_rmdir
   filename_rmdir
   __do_sys_unlinkat

The parent's link count is already validated; do the same for the
directory being removed and fail with -EFSCORRUPTED.

Reported-by: syzbot+8420e667af06b1709c00@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=8420e667af06b1709c00
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
Link: https://patch.msgid.link/20261016115851.14276-1-ngocthang2710.1999@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>