sch_sfq: fix peek() implementation

sch_sfq: fix peek() implementation

Since commit eeaeb068f139 (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 <[email protected]>
CC: Jarek Poplawski <[email protected]>
CC: Patrick McHardy <[email protected]>
CC: Jesper Dangaard Brouer <[email protected]>
Signed-off-by: David S. Miller <[email protected]>