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: c5ad119fb6 ("net: sched: pfifo_fast use skb_array")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_red.c

@@ -233,8 +233,7 @@ static int red_change(struct Qdisc *sch, struct nlattr *opt,
 	q->flags = ctl->flags;
 	q->limit = ctl->limit;
 	if (child) {
-		qdisc_tree_reduce_backlog(q->qdisc, q->qdisc->q.qlen,
-					  q->qdisc->qstats.backlog);
+		qdisc_tree_flush_backlog(q->qdisc);
 		old_child = q->qdisc;
 		q->qdisc = child;
 	}