block-throttle: enable hierarchical throttling even on traditional hierarchy

ECI may have an use case that configuring each device mapper disk
throttling policy just under root blkio cgroup, but actually using them
in different containers.
Since hierarchical throttling is now only supported on cgroup v2 and ECI
uses cgroup v1, so we have to enable hierarchical throttling on cgroup
v1.
This is ported from redhat 7u, and a year ago Jiufei already ported it
to alikernel 4.9 as well. So I think this change should be acceptable.
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NJiufei Xue <jiufei.xue@linux.alibaba.com>

block/blk-throttle.c

@@ -535,7 +535,9 @@ static void throtl_pd_init(struct blkg_policy_data *pd)
 	 * regardless of the position of the group in the hierarchy.
 	 */
 	sq->parent_sq = &td->service_queue;
-	if (cgroup_subsys_on_dfl(io_cgrp_subsys) && blkg->parent)
+
+	/* Enable hierarchical throttling even on traditional hierarchy */
+	if (blkg->parent)
 		sq->parent_sq = &blkg_to_tg(blkg->parent)->service_queue;
 	tg->td = td;
 }