random(4): Discard low entropy inputs

The later fields of the harvest_event structure are predictable and provide
little value to the entropy pool. Only feed in the relatively high entropy
counter and explicit entropy buffer to increase measured input entropy.

See also:
https://people.freebsd.org/~jmg/vbsdcon_2017_ddfreebsdrng_slides.pdf

PR: 222807
Submitted by: W. Dean Freeman <badfilemagic AT gmail.com>
Reviewed by: jmg (earlier version), delphij
Approved by: secteam (delphij)
Obtained from: HBSD 8d809124d563937edd84c9c9d5494406e359c55c
Security: no -- low entropy marginal input has no known negative affect on pool quality
Differential Revision: https://reviews.freebsd.org/D12610