crypto: arm64 - use the new scatterwalk functions

Use scatterwalk_next() which consolidates scatterwalk_clamp() and
scatterwalk_map(), and use scatterwalk_done_src() which consolidates
scatterwalk_unmap(), scatterwalk_advance(), and scatterwalk_done().
Remove unnecessary code that seemed to be intended to advance to the
next sg entry, which is already handled by the scatterwalk functions.
Adjust variable naming slightly to keep things consistent.

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