crypto: scatterwalk - Remove unnecessary advance in scatterwalk_pagedone

The offset advance in scatterwalk_pagedone not only is unnecessary,
but it was also buggy when it was needed by scatterwalk_copychunks.
As the latter has long ago been fixed to call scatterwalk_advance
directly, we can remove this unnecessary offset adjustment.

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