crypto: x86/aegis - use the new scatterwalk functions

In crypto_aegis128_aesni_process_ad(), use scatterwalk_next() which
consolidates scatterwalk_clamp() and scatterwalk_map().  Use
scatterwalk_done_src() which consolidates scatterwalk_unmap(),
scatterwalk_advance(), and scatterwalk_done().

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