crypto: drbg - ensure drbg hmac sha512 is used in FIPS selftests

Update code comment, self test & healthcheck to use HMAC SHA512,
instead of HMAC SHA256. These changes are in dead-code, or FIPS
enabled code-paths only and have not effect on usual kernel builds.

On systems booting in FIPS mode that has the effect of switch sanity
selftest to HMAC sha512 based (which has been the default DRBG).

This patch updates code from 9b7b9468

 ("crypto: DRBG - switch to
HMAC SHA512 DRBG as default DRBG"), but is not interesting to
cherry-pick for stable updates, because it doesn't affect regular
builds, nor has any tangible effect on FIPS certifcation.

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>