The skb_peek function extracts the head element of list_,
without modifying list_. It is important to note that this is
not necessarily a safe operation, as list_ maintains the original
sk_buff, and other operations on list_ may effect it. To be safe,
either disable interrupts using cli(9), call skb_peek, copy
the data necessary to continue, and re-enable interrupts using
sti(9), or use skb_dequeue(9).
Returns a pointer to sk_buff if there is a next element on the list_.
Otherwise, returns NULL.