The GMAC code assumes that dst==src, which causes problems when trying to add
rfc4543(gcm(aes)) test vectors.

So fix this code to work when source and destination buffer are different.
Signed-off-by: NJussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>