sit: fix double free of fb_tunnel_dev on exit

[ No relevant upstream commit. ]

This problem was fixed upstream by commit 9434266f2c64 ("sit: fix use after free
of fb_tunnel_dev").
The upstream patch depends on upstream commit 5e6700b3bf98 ("sit: add support of
x-netns"), which was not backported into 3.10 branch.

First, explain the problem: when the sit module is unloaded, sit_cleanup() is
called.
rmmod sit
=> sit_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 sit tunnels).
  => unregister_pernet_device()
    => unregister_pernet_operations()
      => ops_exit_list()
        => sit_exit_net()
          => sit_destroy_tunnels()
          In this function, no tunnel is found.
          => unregister_netdevice_queue(sitn->fb_tunnel_dev, &list);
We delete the FB device a second time here!

Because we cannot simply remove the second deletion (sit_exit_net() must remove
the FB device when a netns is deleted), we add an rtnl ops which delete all sit
device excepting the FB device and thus we can keep the explicit deletion in
sit_exit_net().

CC: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Willem de Bruijn <willemb@google.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>