crypto: scatterwalk - simplify map and unmap calling convention

Now that the address returned by scatterwalk_map() is always being
stored into the same struct scatter_walk that is passed in, make
scatterwalk_map() do so itself and return void.

Similarly, now that scatterwalk_unmap() is always being passed the
address field within a struct scatter_walk, make scatterwalk_unmap()
take a pointer to struct scatter_walk instead of the address directly.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>