crypto: x86/aes-gcm - simplify GCM hash subkey derivation

Remove a redundant expansion of the AES key, and use rodata for zeroes.
Also rename rfc4106_set_hash_subkey() to aes_gcm_derive_hash_subkey()
because it's used for both versions of AES-GCM, not just RFC4106.

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