dpif-netdev: Reintroduce ref_cnt for dp_netdev_flow

struct dp_netdev_flow used to have a reference counter.
It has been replaced by RCU. Unfortunately RCU is not
enough if we plan to hold a reference to the dp_netdev_flow
for a long time. So this commit reintroduces reference
counting for struct dp_netdev_flow

Subsequent commits make use of it.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
netdev-dpdk: Move to DPDK 1.7.0

With this commit we move our DPDK support to 1.7.0.
DPDK binaries (starting with dpdk 1.7.0) should be linked with --whole-archive
to include pmd drivers

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
netdev-dpdk: Keep calling rte_eth_tx_burst() until it returns 0

rte_eth_tx_burst() _should_ transmit every packet that it is passed unless the
queue is full. Nontheless some implementation of rte_eth_tx_burst (e.g.
ixgbe_xmit_pkts_vec()) does not transmit more than a fixed number (32) of
packets at a time.

With this commit we assume that there's an error only if rte_eth_tx_burst
returns 0.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>