netdev-dpif: Add metadata to dpif-packet.

Today dpif-netdev has single metadat for given batch, since one
batch belongs to one port, but soon packets fro single tunnel ports
can belong to different ports, so we need to have per packet metadata.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>