[NETNS]: The ip6_fib_timer can work with garbage on net namespace stop.

The del_timer() function doesn't guarantee, that the timer callback
is not active by the time it exits.

Thus, the fib6_net_exit() may kfree() all the data, that is required
by the fib6_run_gc(). The race window is tiny, but slab poisoning can
trigger this bug.

Using del_timer_sync() will cure this.
Signed-off-by: NPavel Emelyanov <xemul@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -1543,7 +1543,7 @@ static int fib6_net_init(struct net *net)
 static void fib6_net_exit(struct net *net)
 {
 	rt6_ifdown(net, NULL);
-	del_timer(net->ipv6.ip6_fib_timer);
+	del_timer_sync(net->ipv6.ip6_fib_timer);
 	kfree(net->ipv6.ip6_fib_timer);
 #ifdef CONFIG_IPV6_MULTIPLE_TABLES
 	kfree(net->ipv6.fib6_local_tbl);