Fix a drop_nlink warning in minix_rename

Syzbot found a drop_nlink warning that is triggered by an easy to
detect nlink corruption. This patch adds sanity checks to minix_unlink
and minix_rename 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/20251104143005.3283980-4-jkoolstra@xs4all.nl
Reviewed-by: Jan Kara <jack@suse.cz>
Reported-by: syzbot+a65e824272c5f741247d@syzkaller.appspotmail.com
Closes: https://syzbot.org/bug?extid=a65e824272c5f741247d
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: LATENTBOUNCE

fs/minix/namei.c

@@ -145,6 +145,11 @@ static int minix_unlink(struct inode * dir, struct dentry *dentry)
 	struct minix_dir_entry * de;
 	int err;
 
+	if (inode->i_nlink == 0) {
+		minix_error_inode(inode, "inode has corrupted nlink");
+		return -EFSCORRUPTED;
+	}
+
 	de = minix_find_entry(dentry, &folio);
 	if (!de)
 		return -ENOENT;
@@ -217,6 +222,17 @@ static int minix_rename(struct mnt_idmap *idmap,
 		if (dir_de && !minix_empty_dir(new_inode))
 			goto out_dir;
 
+		err = -EFSCORRUPTED;
+		if (new_inode->i_nlink == 0 || (dir_de && new_inode->i_nlink != 2)) {
+			minix_error_inode(new_inode, "inode has corrupted nlink");
+			goto out_dir;
+		}
+
+		if (dir_de && old_dir->i_nlink <= 2) {
+			minix_error_inode(old_dir, "inode has corrupted nlink");
+			goto out_dir;
+		}
+
 		err = -ENOENT;
 		new_de = minix_find_entry(new_dentry, &new_folio);
 		if (!new_de)