ip6tnl: allow to use rtnl ops on fb tunnel

rtnl ops where introduced by c075b13098b3 ("ip6tnl: advertise tunnel param via
rtnl"), but I forget to assign rtnl ops to fb tunnels.

Now that it is done, we must remove the explicit call to
unregister_netdevice_queue(), because  the fallback tunnel is added to the queue
in ip6_tnl_destroy_tunnels() when checking rtnl_link_ops of all netdevices (this
is valid since commit 0bd8762824e7 ("ip6tnl: add x-netns support")).

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