crypto: s390/aes-gcm - use the new scatterwalk functions

Use scatterwalk_next() which consolidates scatterwalk_clamp() and
scatterwalk_map().  Use scatterwalk_done_src() and
scatterwalk_done_dst() which consolidate scatterwalk_unmap(),
scatterwalk_advance(), and scatterwalk_done().

Besides the new functions being a bit easier to use, this is necessary
because scatterwalk_done() is planned to be removed.

Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Tested-by: Harald Freudenberger <freude@linux.ibm.com>
Cc: Holger Dengler <dengler@linux.ibm.com>
Cc: linux-s390@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>