crypto: aesni - Add GCM_INIT macro

Reduce code duplication by introducting GCM_INIT macro.  This macro
will also be exposed as a function for implementing scatter/gather
support, since INIT only needs to be called once for the full
operation.

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