crypto: gcm - Use GHASH library instead of crypto_ahash

crypto: gcm - Use GHASH library instead of crypto_ahash

Make the "gcm" template access GHASH using the library API instead of
crypto_ahash.  This is much simpler and more efficient, especially given
that all GHASH implementations are synchronous and CPU-based anyway.

Note that this allows "ghash" to be removed from the crypto_ahash (and
crypto_shash) API, which a later commit will do.

This mirrors the similar cleanup that was done with POLYVAL.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20260319061723.1140720-16-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>