crypto: aesni - Fix out-of-bounds access of the data buffer in generic-gcm-aesni

The aesni_gcm_enc/dec functions can access memory before the start of
the data buffer if the length of the data buffer is less than 16 bytes.
This is because they perform the read via a single 16-byte load. This
can potentially result in accessing a page that is not mapped and thus
causing the machine to crash. This patch fixes that by reading the
partial block byte-by-byte and optionally an via 8-byte load if the block
was at least 8 bytes.

Fixes: 0487ccac

 ("crypto: aesni - make non-AVX AES-GCM work with any aadlen")
Cc: <stable@vger.kernel.org>
Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>