msm: dma_test: Initialize newly allocated memory

The MSM_DMA_IOALLOC ioctl command allocates kernel memory and
this memory can be read back using the MSM_DMA_IORBUF ioctl command.
This memory is not zero-initialized and may contain sensitive data.

Change-Id: I8c55d6fe500e7607690b89806715893783eecf9c
Signed-off-by: Srinivasarao P <spathi@codeaurora.org>