dpdk: quad loop dpdk tx and code optimizations

dpdk: quad loop dpdk tx and code optimizations

Beside performance optimization this change
also introduces rte_mbuf validation in dpdk tx code.
If packet is arrving from non-DPDK source like
self-originated packets or non-dpdk devices (tuntap,
af_packet, netmap, pg) it is not anymore exepcted
that it contains valid rte_mbuf metadata unless it is
explictely stated by setting VNET_BUFFER_RTE_MBUF_VALID
flag.

dpdk-input node sets VNET_BUFFER_RTE_MBUF_VALID on all
packets and that reduces cost of validation in tx node.

Change-Id: I4ad40f398f7b5cf90656a2069a27c0f6fc13efba
Signed-off-by: Damjan Marion <damarion@cisco.com>