ip6_tunnel: Add support for wildcard tunnel endpoints.

This patch adds support for tunnels with local or
remote wildcard endpoints. With this we get a
NBMA tunnel mode like we have it for ipv4 and
sit tunnels.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>