ip6tnl: fix double free of fb_tnl_dev on exit

[ No relevant upstream commit. ]

This problem was fixed upstream by commit 1e9f3d6f1c40 ("ip6tnl: fix use after
free of fb_tnl_dev").
The upstream patch depends on upstream commit 0bd8762824e7 ("ip6tnl: add x-netns
support"), which was not backported into 3.10 branch.

First, explain the problem: when the ip6_tunnel module is unloaded,
ip6_tunnel_cleanup() is called.
rmmod ip6_tunnel
=> ip6_tunnel_cleanup()
  => rtnl_link_unregister()
    => __rtnl_kill_links()
      => for_each_netdev(net, dev) {
        if (dev->rtnl_link_ops == ops)
        	ops->dellink(dev, &list_kill);
        }
At this point, the FB device is deleted (and all ip6tnl tunnels).
  => unregister_pernet_device()
    => unregister_pernet_operations()
      => ops_exit_list()
        => ip6_tnl_exit_net()
          => ip6_tnl_destroy_tunnels()
            => t = rtnl_dereference(ip6n->tnls_wc[0]);
               unregister_netdevice_queue(t->dev, &list);
We delete the FB device a second time here!

The previous fix removes these lines, which fix this double free. But the patch
introduces a memory leak when a netns is destroyed, because the FB device is
never deleted. By adding an rtnl ops which delete all ip6tnl device excepting
the FB device, we can keep this exlicit removal in ip6_tnl_destroy_tunnels().

CC: Steven Rostedt <rostedt@goodmis.org>
CC: Willem de Bruijn <willemb@google.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reported-by: Steven Rostedt <srostedt@redhat.com>
Tested-by: Steven Rostedt <srostedt@redhat.com> (and our entire MRG team)
Tested-by: "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>
Tested-by: John Kacur <jkacur@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>