crypto: aesni - Move ghash_mul to GCM_COMPLETE

Prepare to handle partial blocks between scatter/gather calls.
For the last partial block, we only want to calculate the aadhash
in GCM_COMPLETE, and a new partial block macro will handle both
aadhash update and encrypting partial blocks between calls.

Signed-off-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>