sctp: use SCTP_FUTURE_ASSOC and add SCTP_CURRENT_ASSOC for SCTP_DEFAULT_SNDINFO sockopt

Check with SCTP_ALL_ASSOC instead in sctp_setsockopt_default_sndinfo and
check with SCTP_FUTURE_ASSOC instead in sctp_getsockopt_default_sndinfo,
it's compatible with 0.

SCTP_CURRENT_ASSOC is supported for SCTP_DEFAULT_SNDINFO in this patch.

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