sctp: Make sctp_enqueue_event tak an skb list.

Pass this, instead of an event.  Then everything trickles down and we
always have events a non-empty list.

Then we needs a list creating stub to place into .enqueue_event for sctp_stream_interleave_1.

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>