sctp: Always pass skbs on a list to sctp_ulpq_tail_event().

This way we can simplify the logic and remove assumptions
about the implementation of skb lists.

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>