D43768 has two oversights, affecting the non-default pipe calculation
when RFC6675 (net.inet.tcp.sack.revised) is not active.

• by the time CC_RTO is processed, the timeout processing routing

already will have pulled back snd_nxt from whereever it was, down
to snd_una.

• snd_fack is not consistently pulled forward, only during SACK loss

recovery.

While these oversights have the opposite effect (snd_nxt == snd_una
would shrink the pipe, but leaving snd_fack aging (below snd_una),
would increase the non-6675 pipe, the general effect appears to be
an unintended inflated ssthresh.

When the (default) net.inet.tcp.sack.revised pipe calculation is
used, ssthresh will have the intended value (Flight Size * beta)