sch_sfq: fix peek() implementation

Since commit eeaeb068

 (sch_sfq: allow big packets and be fair),
sfq_peek() can return a different skb that would be normally dequeued by
sfq_dequeue() [ if current slot->allot is negative ]

Use generic qdisc_peek_dequeued() instead of custom implementation, to
get consistent result.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Jarek Poplawski <jarkao2@gmail.com>
CC: Patrick McHardy <kaber@trash.net>
CC: Jesper Dangaard Brouer <hawk@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>