crypto: drbg - Initialise mutex in drbg_healthcheck_sanity

As we moved the mutex init out of drbg_instantiate and into cra_init
we need to explicitly initialise the mutex in drbg_healthcheck_sanity.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Stephan Mueller <smueller@chronox.de>