TCP: fix cubic RTO reaction.

Proper TCP Cubic operation requires the knowledge
of the maximum congestion window prior to the
last congestion event.

This restores and improves a bugfix previously added
by jtl@ but subsequently removed due to a revert.

Reported by: chengc_netapp.com
Reviewed by: chengc_netapp.com, tuexen (mentor)
Approved by: tuexen (mentor), rgrimes (mentor)
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D25133