crypto: skcipher - fix mismatch between mapping and unmapping order

Local kunmaps have to be unmapped in the opposite order from which they
were mapped.  My recent change flipped the unmap order in the
SKCIPHER_WALK_DIFF case.  Adjust the mapping side to match.

This fixes a WARN_ON_ONCE that was triggered when running the
crypto-self tests on a 32-bit kernel with
CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP=y.

Fixes: 95dbd711 ("crypto: skcipher - use the new scatterwalk functions")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>