geneve: implement support for IPv6-based tunnels

NOTE: Link-local IPv6 addresses for remote endpoints are not supported,
since the driver currently has no capacity for binding a geneve
interface to a specific link.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Reviewed-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>