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>

Author: LATENTBOUNCE

fs/minix/namei.c

@@ -161,15 +161,24 @@ static int minix_unlink(struct inode * dir, struct dentry *dentry)
 static int minix_rmdir(struct inode * dir, struct dentry *dentry)
 {
 	struct inode * inode = d_inode(dentry);
-	int err = -ENOTEMPTY;
-
-	if (minix_empty_dir(inode)) {
-		err = minix_unlink(dir, dentry);
-		if (!err) {
-			inode_dec_link_count(dir);
-			inode_dec_link_count(inode);
-		}
+	int err = -EFSCORRUPTED;
+
+	if (dir->i_nlink <= 2) {
+		minix_error_inode(dir, "inode has corrupted nlink");
+		goto out;
+	}
+
+	err = -ENOTEMPTY;
+	if (!minix_empty_dir(inode))
+		goto out;
+
+	err = minix_unlink(dir, dentry);
+	if (!err) {
+		inode_dec_link_count(dir);
+		inode_dec_link_count(inode);
 	}
+
+out:
 	return err;
 }