sctp: improve input validation of mapped addresses in send()
Reported by: syzbot+35528f275f2eea6317cc@syzkaller.appspotmail.com
Reported by: syzbot+ac29916d5f16d241553d@syzkaller.appspotmail.com
(cherry picked from commit b732091a761a04c7a04229f28642d27dc48485d8)