crypto: drbg - use memzero_explicit() for clearing sensitive data

Compiler dead store optimization can sometimes remove final calls
to memset() used to clear sensitive data at the end of a function.
Replace trailing memset() calls with memzero_explicit() to
preclude unwanted removal.

Signed-off-by: Nickolaus Woodruff <nickolauswoodruff@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>