sctp: Hold association locks across socket wakeups when freeing

At this point we do not hold the inpcb lock, so the only thing holding
the socket reference live is the TCB lock, which needs to be acquired by
sctp_inpcb_free() in order to destroy associations. Defer the unlock to
until after we dereference the socket reference.

Reported by: syzbot+1d0f2c4675de76a4cf1e@syzkaller.appspotmail.com
Reported by: syzbot+fabee77954fe69d3a5ad@syzkaller.appspotmail.com
Reviewed by: tuexen
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31754