x509: Separately calculate sha256 for blacklist

x509: Separately calculate sha256 for blacklist

Calculate the SHA256 hash for blacklisting purposes independently of the
signature hash (which may be something other than SHA256).

This is necessary because when ML-DSA is used, no digest is calculated.

Note that this represents a change of behaviour in that the hash used for
the blacklist check would previously have been whatever digest was used
for, say, RSA-based signatures.  It may be that this is inadvisable.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
cc: Lukas Wunner <lukas@wunner.de>
cc: Ignat Korchagin <ignat@cloudflare.com>
cc: Stephan Mueller <smueller@chronox.de>
cc: Eric Biggers <ebiggers@kernel.org>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: keyrings@vger.kernel.org
cc: linux-crypto@vger.kernel.org