cryptosoft(4): Zero plain hash contexts, too

An OCF-naive user program could use these primitives to implement HMAC, for
example. This would make the freed context sensitive data.

Probably other bzeros in this file should be explicit_bzeros as well.
Future work.

Reviewed by: jhb, markj
Differential Revision: https://reviews.freebsd.org/D14662 (minor part of a larger work)