crypto: scatterwalk - Add scatterwalk_ffwd helper

This patch adds the scatterwalk_ffwd helper which can create an
SG list that starts in the middle of an existing SG list.  The
new list may either be part of the existing list or be a chain
that latches onto part of the existing list.

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