btrfs: hold a ref on the root in find_data_references

We're looking up the data references for the bytenr in a root, we need
to hold a ref on that root while we're doing that.
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/relocation.c

@@ -3751,7 +3751,11 @@ static int find_data_references(struct reloc_control *rc,
 	root = read_fs_root(fs_info, ref_root);
 	if (IS_ERR(root)) {
 		err = PTR_ERR(root);
-		goto out;
+		goto out_free;
+	}
+	if (!btrfs_grab_fs_root(root)) {
+		err = -ENOENT;
+		goto out_free;
 	}
 
 	key.objectid = ref_objectid;
@@ -3864,6 +3868,8 @@ static int find_data_references(struct reloc_control *rc,
 
 	}
 out:
+	btrfs_put_fs_root(root);
+out_free:
 	btrfs_free_path(path);
 	return err;
 }