gcm_avx_init: zero the ghash state after hashing the IV

IVs != 96 bits get hashed with GHASH to bring them to 96 bits. Any call
to GHASH will mix the ghash state in gcm_ghash. This is expected to be
zero at first use in an encrypt or decrypt operation, so it needs to be
zeroed after using GHASH in setup.

gcm_init() does this, but gcm_avx_init() zeroed it before setup, not
after, resulting in incorrect encrypt/decrypt results when using AVX GCM
with an IV != 96 bits.

OpenZFS _always_ uses a 96 bit IV (ZIO_DATA_IV_LEN) so this will never
have been hit in any real-world use, which is extremely fortunate, as we
would have incorrectly-encrypted data on-disk. Still, as long as we have
this code here we should make sure it's correct.

Thanks-to: Joel Low <joel@joelsplace.sg>
Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Attila Fülöp <attila@fueloep.org>