Fix btrfs_fill_super to return -EINVAL when no FS found

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 9624923..8928649 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -236,9 +236,9 @@
 
 	tree_root = open_ctree(sb, fs_devices);
 
-	if (!tree_root || IS_ERR(tree_root)) {
+	if (IS_ERR(tree_root)) {
 		printk("btrfs: open_ctree failed\n");
-		return -EIO;
+		return PTR_ERR(tree_root);
 	}
 	sb->s_fs_info = tree_root;
 	disk_super = &tree_root->fs_info->super_copy;