Adopt rte_pktmbuf_free_bulk()

[cjdoucette]

More recent versions of DPDK add the ability to do a bulk freeing of packets. We should adopt it into Gatekeeper when we upgrade DPDK.

[AltraMayor]

While working on this issue, one should investigate all calls to rte_pktmbuf_free() since there're a number of opportunities to use rte_pktmbuf_free_bulk() in the code.