tcp: fix handling of <RST,ACK> segments in SYN-RCVD for RACK and BBR

This deals with TCP endpoints in the SYN-RCVD state coming from the
SYN-SENT state.

Reviewed by: rscheff
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D41203