Fix excessive reseed rate in random(4).

[markm]

Summary

Fix braino of mine where the reseeds would happen far too often, making the kernel process way too busy.
PR: 230808
Submitted by: Conrad Meyer <[e-mail]>
Reported by: Danilo Egea Gondolfo <[e-mail]>

Test Plan

Make the PRNG work hard while doing something intensive like "make
world". To make the PRNG work hard, do e.g.
$ dd if=/dev/random of=/dev/null bs=$((1024*1024))

[markm]

Apologies to delphij for abandoning the previous identical-looking commit. I was making a pig's ear of the archanist work.

[delphij]

LGTM from an earlier review.

[cem]

Copied from earlier review D16866:

This one looks obviously correct to me. Go ahead and commit it. It fixes a security weakness in our implementation relative to spec Fortuna, so I'd be sure to tag the commit with "Security: yes."

[markm]

Thanks folks! Tinderbox build going, then I'll commit.