sctp: Pass sk_buff_head explicitly to sctp_ulpq_tail_event().

Now the SKB list implementation assumption can be removed.

And now that we know that the list head is always non-NULL
we can remove the code blocks dealing with that as well.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>