ip_tunnels: add IPv6 addresses to ip_tunnel_key

Add the IPv6 addresses as an union with IPv4 ones. When using IPv4, the
newly introduced padding after the IPv4 addresses needs to be zeroed out.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>