openvswitch: netlink attributes for IPv6 tunneling

Add netlink attributes for IPv6 tunnel addresses. This enables IPv6 support
for tunnels.

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