MFC r367007: tcp: move cwnd and ssthresh updates into cc modules

This will pave the way of setting ssthresh differently in TCP CUBIC, according
to RFC8312 section 4.7.

Use dynamic tcp_maxseg() adjusting for tcp options instead of static t_maxseg.

Submitted by: chengc_netapp.com
Reviewed by: rrs, tuexen, rscheff
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26807