Limit the amount of "fast" entropy. We don't need nearly as much
for security, and the excess just slows things down badly.

PR: 230808
Submitted by: rwmaillists@googlemail.com, but tweeked by me
Reported by: Danilo Egea Gondolfo <danilo@FreeBSD.org>
Reviewed by: cem,delphij
Approved by: re(rgrimes)
Approved by: so(delphij)
MFC after: 1 Month
Differential Revision: https://reviews.freebsd.org/D16873