fix log of expired ddl merge task

src/storage/ddl/ob_ddl_merge_task.cpp

@@ -372,6 +372,7 @@ int ObDDLTableMergeTask::process()
     } else if (OB_FAIL(ddl_kv_mgr_handle.get_obj()->get_ddl_param(ddl_param))) {
       LOG_WARN("get tablet ddl param failed", K(ret));
     } else if (merge_param_.start_scn_ > SCN::min_scn() && merge_param_.start_scn_ < ddl_param.start_scn_) {
+      ret = OB_TASK_EXPIRED;
       LOG_INFO("ddl merge task expired, do nothing", K(merge_param_), "new_start_scn", ddl_param.start_scn_);
     } else if (merge_param_.is_commit_ && OB_FAIL(ObTabletDDLUtil::check_data_integrity(ddl_sstable_handles,
                                                                                         ddl_param.start_scn_,