net: sit: Unregister catch-all devices

A sit interface created without a local or a remote address is linked
into the `sit_net::tunnels_wc` list of its original namespace. When
deleting a network namespace, delete the devices that have been moved.

The following script triggers a null pointer dereference if devices
linked in a deleted `sit_net` remain:

    for i in `seq 1 30`; do
        ip netns add ns-test
        ip netns exec ns-test ip link add dev veth0 type veth peer veth1
        ip netns exec ns-test ip link add dev sit$i type sit dev veth0
        ip netns exec ns-test ip link set dev sit$i netns $$
        ip netns del ns-test
    done
    for i in `seq 1 30`; do
        ip link del dev sit$i
    done

Fixes: 5e6700b3 ("sit: add support of x-netns")
Signed-off-by: NHristo Venev <hristo@venev.name>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -1867,9 +1867,9 @@ static void __net_exit sit_destroy_tunnels(struct net *net,
 		if (dev->rtnl_link_ops == &sit_link_ops)
 			unregister_netdevice_queue(dev, head);
 
-	for (prio = 1; prio < 4; prio++) {
+	for (prio = 0; prio < 4; prio++) {
 		int h;
-		for (h = 0; h < IP6_SIT_HASH_SIZE; h++) {
+		for (h = 0; h < (prio ? IP6_SIT_HASH_SIZE : 1); h++) {
 			struct ip_tunnel *t;
 
 			t = rtnl_dereference(sitn->tunnels[prio][h]);