[CRYPTO] api: Fixed crypto_tfm context alignment

Previously the __aligned__ attribute was added to the crypto_tfm context
member to ensure it is alinged correctly on architectures such as arm.
Unfortunately kmalloc does not use the same minimum alignment rules as
gcc so this is useless.

This patch changes it to use kmalloc's minimum alignment.

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