ip6tnl/rtnl: add IFLA_IPTUN_PROTO on dump

IPv6 tunnels can have three mode: 4in6, 6in6 and xin6.
This information was missing in the netlink message.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>