openvswitch: Move tunnel destroy function to oppenvswitch module.

This function will be used in gre and geneve vport implementations.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>