sctp: Bring SCTP_MAXSEG socket option into ietf API extension compliance

Brings maxseg socket option set/get into line with the latest ietf socket
extensions API draft, while maintaining backwards compatibility.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>