sctp: improve shutting down the read side of a socket

When shutdown(..., SHUT_RD) or shutdown(..., SHUT_RDWR) is called,
really clean up the read queue and issue an ungraceful shutdown if
user messages are affected.

Reported by: syzbot+d4e1d30d578891245f59@syzkaller.appspotmail.com
MFC after: 3 days