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>