fsverity: Switch from crypto_shash to SHA-2 library

fsverity: Switch from crypto_shash to SHA-2 library

fsverity supports two hash algorithms: SHA-256 and SHA-512.  Since both
of these have a library API now, just use the library API instead of
crypto_shash.  Even with multiple algorithms, the library-based code
still ends up being quite a bit simpler, due to how clumsy the
old-school crypto API is.  The library-based code is also more
efficient, since it avoids overheads such as indirect calls.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250630172224.46909-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>