pkt_sched: Add qdisc->ops->peek() implementation.

Add qdisc->ops->peek() implementation for work-conserving qdiscs.
With feedback from Patrick McHardy.

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_blackhole.c

@@ -33,6 +33,7 @@ static struct Qdisc_ops blackhole_qdisc_ops __read_mostly = {
 	.priv_size	= 0,
 	.enqueue	= blackhole_enqueue,
 	.dequeue	= blackhole_dequeue,
+	.peek		= blackhole_dequeue,
 	.owner		= THIS_MODULE,
 };