Fix a drop_nlink warning in minix_rmdir

Fix a drop_nlink warning in minix_rmdir

Syzbot found a drop_nlink warning that is triggered by an easy to
detect nlink corruption of a directory. This patch adds a sanity check
to minix_rmdir to prevent the warning and instead return EFSCORRUPTED to
the caller.

The changes were tested using the syzbot reproducer as well as local
testing.

Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
Link: https://patch.msgid.link/20251104143005.3283980-3-jkoolstra@xs4all.nl
Reviewed-by: Jan Kara <jack@suse.cz>
Reported-by: syzbot+4e49728ec1cbaf3b91d2@syzkaller.appspotmail.com
Closes: https://syzbot.org/bug?extid=4e49728ec1cbaf3b91d2
Signed-off-by: Christian Brauner <brauner@kernel.org>