btrfs: Error handle for get_ref_objectid_v0() in relocate_block_group()

We need error checking code for get_ref_objectid_v0() in
relocate_block_group(), to avoid unpredictable result, especially
for accessing uninitialized value(when function failed) after
this line.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/relocation.c

@@ -3976,6 +3976,10 @@ restart:
 			       sizeof(struct btrfs_extent_item_v0));
 			ret = get_ref_objectid_v0(rc, path, &key, &ref_owner,
 						  &path_change);
+			if (ret < 0) {
+				err = ret;
+				break;
+			}
 			if (ref_owner < BTRFS_FIRST_FREE_OBJECTID)
 				flags = BTRFS_EXTENT_FLAG_TREE_BLOCK;
 			else