tcp: cubic - restart epoch after RTO

This is a migitation to avoid sudden extreme jumps in
cwnd, as t_epoch can be very out of date after an RTO.
Per RFC9438, sec 4.8, t_epoch is to be reset whenever
cwnd grows beyond ssthresh (CC phase transitions from
slow start to congestion avoidance), to be fixed with
the upcoming cc_cubic changes.

MFC after: 3 days
Reviewed By: cc, transport
Sponsored by: NetApp, Inc
Differential Revision: https://reviews.freebsd.org/D44023

(cherry picked from commit 038699a8f18a0a651ee06b85fa1dbbee1eab56f1)