TCP rack does not work properly with cubic.

Right now if you use rack with cubic (the new default cc) you will have
improper results. This is because rack uses different variables than
the base stack (or bbr) and thus tcp_compute_pipe() always returns
so that cubic will choose a 30% backoff not the 50% backoff it should
when it is newreno compatibility mode. The fix is to allow a stack (rack)
to override its own compute_pipe.

Reviewed by: tuexen, rscheff
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D36711