crypto: aesni - clean up mapping of associated data

The gcm(aes-ni) driver is only built for x86_64, which does not make
use of highmem. So testing for PageHighMem is pointless and can be
omitted.

While at it, replace GFP_ATOMIC with the appropriate runtime decided
value based on the context.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>