net_sched: prio: use qdisc_dequeue_peeked

commit 07bd8df5
(sch_sfq: fix peek() implementation) changed sfq to use generic
peek helper.

This makes HFSC complain about a non-work-conserving child qdisc, if
prio with sfq child is used within hfsc:

hfsc peeks into prio qdisc, which will then peek into sfq.
returned skb is stashed in sch->gso_skb.

Next, hfsc tries to dequeue from prio, but prio will call sfq dequeue
directly, which may return NULL instead of previously peeked-at skb.

Have prio call qdisc_dequeue_peeked, so sfq->dequeue() is
not called in this case.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>