tcp: Add PRR cwnd reduction for non-SACK loss

This completes PRR cwnd reduction in all circumstances
for the base TCP stack (SACK loss recovery, ECN window reduction,
non-SACK loss recovery), preventing the arriving ACKs to
clock out new data at the old, too high rate. This
reduces the chance to induce additional losses while
recovering from loss (during congested network conditions).

For non-SACK loss recovery, each ACK is assumed to have
one MSS delivered. In order to prevent ACK-split attacks,
only one window worth of ACKs is considered to actually
have delivered new data.

MFC after: 6 weeks
Reviewed By: rrs, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29441