sctp: use SCTP_FUTURE_ASSOC and add SCTP_CURRENT_ASSOC for SCTP_EVENT sockopt

Check with SCTP_ALL_ASSOC instead in sctp_setsockopt_event and
check with SCTP_FUTURE_ASSOC instead in sctp_getsockopt_event,
it's compatible with 0.

SCTP_CURRENT_ASSOC is supported for SCTP_EVENT in this patch.

It also adds sctp_assoc_ulpevent_type_set() to make code more
readable.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>