crypto: shash - Fix digest size offset

When an shash algorithm is exported as ahash, ahash will access
its digest size through hash_alg_common.  That's why the shash
layout needs to match hash_alg_common.  This wasn't the case
because the alignment weren't identical.

This patch fixes the problem.

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