TCP Cubic: recalculate cwnd for every ACK.

Since cubic calculates cwnd based on absolute
time, retaining RFC3465 (ABC) once-per-window updates
can lead to dramatic changes of cwnd in the convex
region. Updating cwnd for each incoming ack minimizes
this delta, preventing unintentional line-rate bursts.

Reviewed by: chengc_netapp.com, tuexen (mentor)
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26060