crypto: gcm - Convert to new AEAD interface

This patch converts generic gcm and its associated transforms to
the new AEAD interface.  The biggest reward is in code reduction
for rfc4543 where it used to do IV stitching which is no longer
needed as the IV is already part of the AD on input.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>