sctp: use SCTP_FUTURE_ASSOC for SCTP_MAXSEG sockopt

Check with SCTP_FUTURE_ASSOC instead in
sctp_set/getsockopt_maxseg, it's compatible with 0.
Also check asoc_id early as other sctp setsockopts does.

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