btrfs: hold a ref on the root in resolve_indirect_ref

We're looking up a random root, we need to hold a ref on it while we're
using it.
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -524,7 +524,13 @@ static int resolve_indirect_ref(struct btrfs_fs_info *fs_info,
 	if (IS_ERR(root)) {
 		srcu_read_unlock(&fs_info->subvol_srcu, index);
 		ret = PTR_ERR(root);
-		goto out;
+		goto out_free;
+	}
+
+	if (!btrfs_grab_fs_root(root)) {
+		srcu_read_unlock(&fs_info->subvol_srcu, index);
+		ret = -ENOENT;
+		goto out_free;
 	}
 
 	if (btrfs_is_testing(fs_info)) {
@@ -577,6 +583,8 @@ static int resolve_indirect_ref(struct btrfs_fs_info *fs_info,
 	ret = add_all_parents(root, path, parents, ref, level, time_seq,
 			      extent_item_pos, total_refs, ignore_offset);
 out:
+	btrfs_put_fs_root(root);
+out_free:
 	path->lowest_level = 0;
 	btrfs_release_path(path);
 	return ret;