This patch fix two issues:

* If kernel is not supporting 6rd then ioctl() call
  will return EINVAL, if so just skip perror call.

* 6rd scope is ipv6/ip tunnels. Dont try to fetch
  6rd tunnel parms if tunnel protocol != IPPROTO_IPV6.
Signed-off-by: Alexandre Cassen <acassen@freebox.fr>