[SKB]: __skb_dequeue = skb_peek + __skb_unlink

By rearranging the order of declarations, __skb_dequeue() is expressed in terms of

 * skb_peek() and
 * __skb_unlink(),

thus in effect mirroring the analogue implementation of __skb_dequeue_tail().

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>