crypto: drbg - Simplify "uninstantiate" logic

drbg_kcapi_seed() calls drbg_uninstantiate() only to free drbg->jent and
set drbg->instantiated = false.  However, the latter is necessary only
because drbg_kcapi_seed() sets drbg->instantiated = true too early.  Fix
that, then just inline the freeing of drbg->jent.

Then, simplify the actual "uninstantiate" in drbg_kcapi_exit().  Just
free drbg->jent (note that this is a no-op on error and null pointers),
then memzero_explicit() the entire drbg_state.

Note that in reality the memzero_explicit() is redundant, since the
crypto_rng API zeroizes the memory anyway.  But the way SP800-90A is
worded, it's easy to imagine that someone assessing conformance with it
would be looking for code in drbg.c that says it does an "Uninstantiate"
and does the zeroization.  So it's probably worth keeping it somewhat
explicit, even though that means double zeroization in practice.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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