sit: fix double free of fb_tunnel_dev on exit

[ No relevant upstream commit. ]

This problem was fixed upstream by commit 9434266f ("sit: fix use after free
of fb_tunnel_dev").
The upstream patch depends on upstream commit 5e6700b3 ("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>

net/ipv6/sit.c

@@ -1507,6 +1507,15 @@ static const struct nla_policy ipip6_policy[IFLA_IPTUN_MAX + 1] = {
 #endif
 };
 
+static void ipip6_dellink(struct net_device *dev, struct list_head *head)
+{
+	struct net *net = dev_net(dev);
+	struct sit_net *sitn = net_generic(net, sit_net_id);
+
+	if (dev != sitn->fb_tunnel_dev)
+		unregister_netdevice_queue(dev, head);
+}
+
 static struct rtnl_link_ops sit_link_ops __read_mostly = {
 	.kind		= "sit",
 	.maxtype	= IFLA_IPTUN_MAX,
@@ -1517,6 +1526,7 @@ static struct rtnl_link_ops sit_link_ops __read_mostly = {
 	.changelink	= ipip6_changelink,
 	.get_size	= ipip6_get_size,
 	.fill_info	= ipip6_fill_info,
+	.dellink	= ipip6_dellink,
 };
 
 static struct xfrm_tunnel sit_handler __read_mostly = {