crypto: s390 - fix aes,des ctr mode concurrency finding.

An additional testcase found an issue with the last
series of patches applied: the fallback solution may
not save the iv value after operation. This very small
fix just makes sure the iv is copied back to the
walk/desc struct.

Cc: <stable@vger.kernel.org> # 3.14+
Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>