vxlan: Extend vxlan handlers for openvswitch.

Following patch adds data field to vxlan socket and export
vxlan handler api.
vh->data is required to store private data per vxlan handler.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>