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

src/vlib/dma/dma.h

@@ -38,6 +38,7 @@ typedef struct
     };
     u32 features;
   };
+  u16 max_batches;
   u16 max_transfers;
   u32 max_transfer_size;
   vlib_dma_batch_callback_fn *callback_fn;