sctp: Avoid unnecessary refcount bumps in sctp_inpcb_bind()

We only drop the inp lock when binding to a specific port. So, only
acquire an extra reference when required. This simplifies error
handling a bit.

Reviewed by: tuexen
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31732