Aad requires padding with zeroes up to 15 bytes in some cases. This
patch increases the allocated buffer size for aad and prevents the
driver accessing uninitialized memory region.

v1->v2: Specify reason for alloc size change in descriptions.
Signed-off-by: Jia Jie Ho <jiajie.ho@starfivetech.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>