tcp: simplify stack switching protocol

Before this patch, a stack (tfb) accepts a tcpcb (tp), if the
tp->t_state is TCPS_CLOSED or tfb->tfb_tcp_handoff_ok is not NULL
and tfb->tfb_tcp_handoff_ok(tp) returns 0.
After this patch, the only check is tfb->tfb_tcp_handoff_ok(tp)
returns 0. tfb->tfb_tcp_handoff_ok must always be provided.
For existing TCP stacks (FreeBSD, RACK and BBR) there is no
functional change. However, the logic is simpler.

Reviewed by: lstewart, peter_lei_ieee_.org, rrs
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D45253