sctp: introduce SCTP_FUTURE/CURRENT/ALL_ASSOC

This patch is to add 3 constants SCTP_FUTURE_ASSOC,
SCTP_CURRENT_ASSOC and SCTP_ALL_ASSOC for reserved
assoc_ids, as defined in rfc6458#section-7.2.

And add the process for them when doing lookup and
inserting in sctp_id2assoc and sctp_assoc_set_id.

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