Revert "fsverity: relax build time dependency on CRYPTO_SHA256"

Revert "fsverity: relax build time dependency on CRYPTO_SHA256"

This reverts commit e3a606f2c544b231f6079c8c5fea451e772e1139 because it
allows people to create broken configurations that enable FS_VERITY but
not SHA-256 support.

The commit did allow people to disable the generic SHA-256
implementation when it's not needed.  But that at best allowed saving a
bit of code.  In the real world people are unlikely to intentionally and
correctly make such a tweak anyway, as they tend to just be confused by
what all the different crypto kconfig options mean.

Of course we really need the crypto API to enable the correct
implementations automatically, but that's for a later fix.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://lore.kernel.org/r/20250217185105.26751-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>