This patch converts rfc4106-gcm-aesni to the new AEAD interface.
The low-level interface remains as is for now because we can't
touch it until cryptd itself is upgraded.

In the conversion I've also removed the duplicate copy of the
context in the top-level algorithm.  Now all processing is carried
out in the low-level __driver-gcm-aes-aesni algorithm.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>