vlib: pre-alloc dma backend batches

vlib: pre-alloc dma backend batches

Allocate and initialize dma batch structure when adding dma config.
The number of required dma batches is set by max_batches parameter.
Thus dma batches are not allocated dynamically in worker thread.
Application need to check the return value of vlib_dma_batch_new.

Type: improvement

Signed-off-by: Marvin Liu <yong.liu@intel.com>
Change-Id: I5d05a67b59634cf2862a377d5ab77cb1040343ce