dmaengine: dmatest: Remove use of VLAs

There's an ongoing effort to remove VLAs from the kernel
(https://lkml.org/lkml/2018/3/7/621

) to eventually turn on -Wvla.
The test already pre-allocates some buffers with kmalloc so turn
the two VLAs in to pre-allocated kmalloc buffers.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Sinan Kaya <okaya@codeaurora.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>