sctp: Use helper for sctp_ulpq_tail_event() when hooked up to ->enqueue_event

This way we can make sure events sent this way to
sctp_ulpq_tail_event() are on a list as well.  Now all such code paths
are fully covered.

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>