sctp: use SCTP_FUTURE_ASSOC and add SCTP_CURRENT_ASSOC for SCTP_CONTEXT sockopt

Check with SCTP_ALL_ASSOC instead in sctp_setsockopt_context and
check with SCTP_FUTURE_ASSOC instead in sctp_getsockopt_context,
it's compatible with 0.

SCTP_CURRENT_ASSOC is supported for SCTP_CONTEXT in this patch.
It also adjusts some code to keep a same check form as other
functions.

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