ioatdma: Fix bug in selftest after removal of DMA_MEMSET.

Commit 48a9db46

 (3.11) removed the memset op in the xor selftest for ioatdma.
The issue is that with the removal of that op, it never replaced the memset
with a CPU memset. The memory being operated on is expected to be zeroes but
was not. This is causing the xor selftest to fail.

Cc: <stable@vger.kernel.org>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>