crypto: testmgr - Fix DMA-API warning

With DMA-API debug enabled testmgr triggers a "DMA-API: device driver maps memory from stack" warning, when tested on a crypto HW accelerator.

Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>