Extend tbrsize heuristic in pfctl(8) to provide a sensible value for
higher bandwidth interfaces. The new value is used above 2.5 Gbps,
which is the highest standard rate that could be used prior to
r338209, so the default behavior for all existing systems should
remain the same.

The value of 128 chosen is a balance between being big enough to
reduce potential precision/quantization effects stemming from frequent
bucket refills over small time intervals and being small enough to
prevent a greedy driver from burst dequeuing more packets than it has
available hardware ring slots for whenever altq transitions from idle
to backlogged.

Reviewed by: jmallett, kp
MFC after: 2 weeks
Sponsored by: RG Nets
Differential Revision: https://reviews.freebsd.org/D16852