drivers/dma/dmatest.c: switch a GFP_ATOMIC to GFP_KERNEL

It was needlessly using the unreliable GFP_ATOMIC.

Cc: Timur Tabi <timur@freescale.com>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>