net_sched: always reset qdisc backlog in qdisc_reset()

SKB stored in qdisc->gso_skb also counted into backlog.

Some qdiscs don't reset backlog to zero in ->reset(),
for example sfq just dequeue and free all queued skb.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 2ccccf5fb43f ("net_sched: update hierarchical backlog too")
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -685,6 +685,7 @@ void qdisc_reset(struct Qdisc *qdisc)
 		qdisc->gso_skb = NULL;
 	}
 	qdisc->q.qlen = 0;
+	qdisc->qstats.backlog = 0;
 }
 EXPORT_SYMBOL(qdisc_reset);