crypto: aesni - Split AAD hash calculation to separate macro

AAD hash only needs to be calculated once for each scatter/gather operation.
Move it to its own macro, and call it from GCM_INIT instead of
INITIAL_BLOCKS.

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