openvswitch: Retrieve tunnel metadata when receiving from vport-netdev

Retrieve the tunnel metadata for packets received by a net_device and
provide it to ovs_vport_receive() for flow key extraction.

[This hunk was in the GRE patch in the initial series and missed the
 cut for the initial submission for merging.]

Fixes: 614732ea ("openvswitch: Use regular VXLAN net_device device")
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>