squashfs: fix warn_on when root inode is corrupted

Fix warn_on triggered by mounting a fsfuzzer corrupted file system, where
the root inode has been corrupted.
Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>
Reported-by: Steve Grubb <sgrubb@redhat.com>

fs/squashfs/super.c

@@ -275,7 +275,8 @@ static int squashfs_fill_super(struct super_block *sb, void *data, int silent)
 
 	err = squashfs_read_inode(root, root_inode);
 	if (err) {
-		iget_failed(root);
+		make_bad_inode(root);
+		iput(root);
 		goto failed_mount;
 	}
 	insert_inode_hash(root);