net_sched: sch_fq: do not call fq_peek() twice per packet

This refactors the code to not call fq_peek() from fq_dequeue_head()
since the caller can provide the skb.

Also rename fq_dequeue_head() to fq_dequeue_skb() because 'head' is
a bit vague, given the skb could come from t_root rb-tree.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>