crypto: sun4i-ss - fix kmap usage

crypto: sun4i-ss - fix kmap usage

With the recent kmap change, some tests which were conditional on
CONFIG_DEBUG_HIGHMEM now are enabled by default.
This permit to detect a problem in sun4i-ss usage of kmap.

sun4i-ss uses two kmap via sg_miter (one for input, one for output), but
using two kmap at the same time is hard:
"the ordering has to be correct and with sg_miter that's probably hard to get
right." (quoting Tlgx)

So the easiest solution is to never have two sg_miter/kmap open at the same time.
After each use of sg_miter, I store the current index, for being able to
resume sg_miter to the right place.

Fixes: 6298e948215f ("crypto: sunxi-ss - Add Allwinner Security System crypto accelerator")
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>