sctp: improve handling of send() when association is shutdown

Accept send() calls only when the association is not being
shut down or the expicit message EOR mode is used and the
application provides follow-up data.

Reported by: syzbot+341e9ebd9d24ca7dc62a@syzkaller.appspotmail.com

(cherry picked from commit 64b297e803bd8123bfef3fecaa1f8ceae9eea0e6)