net: sched: introduce and use qdisc tree flush/purge helpers

The same code to flush qdisc tree and purge the qdisc queue
is duplicated in many places and in most cases it does not
respect NOLOCK qdisc: the global backlog len is used and the
per CPU values are ignored.

This change addresses the above, factoring-out the relevant
code and using the helpers introduced by the previous patch
to fetch the correct backlog len.

Fixes: c5ad119fb6c0 ("net: sched: pfifo_fast use skb_array")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/sch_cbq.c b/net/sched/sch_cbq.c
index 651879c..114b904 100644
--- a/net/sched/sch_cbq.c
+++ b/net/sched/sch_cbq.c
@@ -1667,17 +1667,13 @@ static int cbq_delete(struct Qdisc *sch, unsigned long arg)
 {
 	struct cbq_sched_data *q = qdisc_priv(sch);
 	struct cbq_class *cl = (struct cbq_class *)arg;
-	unsigned int qlen, backlog;
 
 	if (cl->filters || cl->children || cl == &q->link)
 		return -EBUSY;
 
 	sch_tree_lock(sch);
 
-	qlen = cl->q->q.qlen;
-	backlog = cl->q->qstats.backlog;
-	qdisc_reset(cl->q);
-	qdisc_tree_reduce_backlog(cl->q, qlen, backlog);
+	qdisc_purge_queue(cl->q);
 
 	if (cl->next_alive)
 		cbq_deactivate_class(cl);