crypto: aes/fixed-time - align key schedule with other implementations

crypto: aes/fixed-time - align key schedule with other implementations

The fixed time AES code mangles the key schedule so that xoring the
first round key with values at fixed offsets across the Sbox produces
the correct value. This primes the D-cache with the entire Sbox before
any data dependent lookups are done, making it more difficult to infer
key bits from timing variances when the plaintext is known.

The downside of this approach is that it renders the key schedule
incompatible with other implementations of AES in the kernel, which
makes it cumbersome to use this implementation as a fallback for SIMD
based AES in contexts where this is not allowed.

So let's tweak the fixed Sbox indexes so that they add up to zero under
the xor operation. While at it, increase the granularity to 16 bytes so
we cover the entire Sbox even on systems with 16 byte cachelines.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>