crypto: x86/aes-gcm - use the new scatterwalk functions

In gcm_process_assoc(), use scatterwalk_next() which consolidates
scatterwalk_clamp() and scatterwalk_map().  Use scatterwalk_done_src()
which consolidates scatterwalk_unmap(), scatterwalk_advance(), and
scatterwalk_done().

Also rename some variables to avoid implying that anything is actually
mapped (it's not), or that the loop is going page by page (it is for
now, but nothing actually requires that to be the case).

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