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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 93908fce7280b1146bbc5135b78829e8f8ff1b74)