kern: random: collect ~16x less from fast-entropy sources

Previously, we were collecting at a base rate of:

64 bits x 32 pools x 10 Hz = 2.5 kB/s

This change drops it to closer to 64-ish bits per pool per second, to
work a little better with entropy providers in virtualized environments
without compromising the security goals of Fortuna.

Reviewed by: csprng (cem, delphij, markm)
Differential Revision: https://reviews.freebsd.org/D32021