crypto: scatterwalk - Add no-copy support to copychunks

The function ablkcipher_done_slow is pretty much identical to
scatterwalk_copychunks except that it doesn't actually copy as
the processing hasn't been completed yet.

This patch allows scatterwalk_copychunks to be used in this case
by specifying out == 2.

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