crypto: scatterwalk - Add scatterwalk_crypto_chain helper

A lot of crypto algorithms implement their own chaining function.
So add a generic one that can be used from all the algorithms that
need scatterlist chaining.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>