crypto: seed - stop using cra_alignmask

Instead of specifying a nonzero alignmask, use the unaligned access
helpers.  This eliminates unnecessary alignment operations on most CPUs,
which can handle unaligned accesses efficiently, and brings us a step
closer to eventually removing support for the alignmask field.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>