crypto: scatterwalk - Change scatterwalk_next calling convention

Rather than returning the address and storing the length into an
argument pointer, add an address field to the walk struct and use
that to store the address.  The length is returned directly.

Change the done functions to use this stored address instead of
getting them from the caller.

Split the address into two using a union.  The user should only
access the const version so that it is never changed.

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