Because hardware will read in multiples of 4 SG entries, ensure
the allocated length is always padded.  This was already done
by some callers of ahash_edesc_alloc, but ahash_digest was conspicuously
missing.

In any case, doing it in the allocation function ensures that the
memory is always there.
Reported-by: Guangwu Zhang <guazhang@redhat.com>
Fixes: a5e5c133 ("crypto: caam - fix S/G table passing page boundary")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>