crypto: aesni - don't use GFP_ATOMIC allocation if the request doesn't cross a page in gcm

This patch fixes gcmaes_crypt_by_sg so that it won't use memory
allocation if the data doesn't cross a page boundary.

Authenticated encryption may be used by dm-crypt. If the encryption or
decryption fails, it would result in I/O error and filesystem corruption.
The function gcmaes_crypt_by_sg is using GFP_ATOMIC allocation that can
fail anytime. This patch fixes the logic so that it won't attempt the
failing allocation if the data doesn't cross a page boundary.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>