crypto: scatterwalk - Use sg_chain_ptr on chain entries

Now that scatterwalk_sg_chain sets the chain pointer bit the sg_page
call in scatterwalk_sg_next hits a BUG_ON when CONFIG_DEBUG_SG is
enabled. Use sg_chain_ptr instead of sg_page on a chain entry.

Cc: stable@vger.kernel.org
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>