pkt_sched: sch_generic: Add Qdisc_ops peek() method.

Add Qdisc_ops peek() method in order to replace requeuing.

Based on ideas and patches of Herbert Xu, Patrick McHardy and
David S. Miller.
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -111,6 +111,7 @@ struct Qdisc_ops
 
 	int 			(*enqueue)(struct sk_buff *, struct Qdisc *);
 	struct sk_buff *	(*dequeue)(struct Qdisc *);
+	struct sk_buff *	(*peek)(struct Qdisc *);
 	int 			(*requeue)(struct sk_buff *, struct Qdisc *);
 	unsigned int		(*drop)(struct Qdisc *);