dpdk: mempool priv intialization must be done before releasing buffers to pool

dpdk: mempool priv intialization must be done before releasing buffers to pool

 - Currently mempool priv size is getting initialized after releasing buffers
   to pool. This is causing mismatch in expected & real metadata size value
   and buffers are getting released with wrong offset. (when metadata offset
   is in use for a given platform)
 - Since private data size is 0 initially, metadata size don't include space
   for VLIB_BUFFER_HDR.

Change-Id: I780c4d518104631a3dcf192185bacf58b3598e65
Signed-off-by: Sachin Saxena <sachin.saxena@nxp.com>