Btrfs: print error messages when failing to read trees

When mount fails to read trees like fs tree, checksum tree, extent
tree, etc, there is not enough information about where went wrong.

With this, messages like

"BTRFS warning (device sdf): failed to read root (objectid=7): -5"

would help us a bit.
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -2383,23 +2383,29 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
 	location.offset = 0;
 
 	root = btrfs_read_tree_root(tree_root, &location);
-	if (IS_ERR(root))
-		return PTR_ERR(root);
+	if (IS_ERR(root)) {
+		ret = PTR_ERR(root);
+		goto out;
+	}
 	set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
 	fs_info->extent_root = root;
 
 	location.objectid = BTRFS_DEV_TREE_OBJECTID;
 	root = btrfs_read_tree_root(tree_root, &location);
-	if (IS_ERR(root))
-		return PTR_ERR(root);
+	if (IS_ERR(root)) {
+		ret = PTR_ERR(root);
+		goto out;
+	}
 	set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
 	fs_info->dev_root = root;
 	btrfs_init_devices_late(fs_info);
 
 	location.objectid = BTRFS_CSUM_TREE_OBJECTID;
 	root = btrfs_read_tree_root(tree_root, &location);
-	if (IS_ERR(root))
-		return PTR_ERR(root);
+	if (IS_ERR(root)) {
+		ret = PTR_ERR(root);
+		goto out;
+	}
 	set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
 	fs_info->csum_root = root;
 
@@ -2416,7 +2422,7 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
 	if (IS_ERR(root)) {
 		ret = PTR_ERR(root);
 		if (ret != -ENOENT)
-			return ret;
+			goto out;
 	} else {
 		set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
 		fs_info->uuid_root = root;
@@ -2425,13 +2431,19 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
 	if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
 		location.objectid = BTRFS_FREE_SPACE_TREE_OBJECTID;
 		root = btrfs_read_tree_root(tree_root, &location);
-		if (IS_ERR(root))
-			return PTR_ERR(root);
+		if (IS_ERR(root)) {
+			ret = PTR_ERR(root);
+			goto out;
+		}
 		set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
 		fs_info->free_space_root = root;
 	}
 
 	return 0;
+out:
+	btrfs_warn(fs_info, "failed to read root (objectid=%llu): %d",
+		   location.objectid, ret);
+	return ret;
 }
 
 int open_ctree(struct super_block *sb,
@@ -3004,6 +3016,7 @@ int open_ctree(struct super_block *sb,
 	fs_info->fs_root = btrfs_read_fs_root_no_name(fs_info, &location);
 	if (IS_ERR(fs_info->fs_root)) {
 		err = PTR_ERR(fs_info->fs_root);
+		btrfs_warn(fs_info, "failed to read fs tree: %d", err);
 		goto fail_qgroup;
 	}