sctp: Fix racy UNBOUND flag check in sctp_inpcb_bind()

SCTP needs to avoid binding a given socket twice. The check used to
avoid this is racy since neither the inpcb lock nor the global info lock
is held. Fix it by synchronizing using the global info lock. In
particular, sctp_inpcb_bind() may drop the inpcb lock in some cases, but
the info lock is sufficient to prevent double insertion into PCB hash
tables.

Reported by: syzbot+548a8560d959669d0e12@syzkaller.appspotmail.com
Reviewed by: tuexen
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31734