sctp: cleanup the SCTP_MAXSEG socket option.
This patch makes the handling of the SCTP_MAXSEG socket option
compliant with RFC 6458 (SCTP socket API) and fixes an issue
found by syzkaller.
Reported by: syzbot+a2791b89ab99121e3333@syzkaller.appspotmail.com
(cherry picked from commit 989453da0589b8dc5c1948fd81f986a37ea385eb)