[NET]: Fix gen_estimator timer removal race

As noticed by Jarek Poplawski <jarkao2@o2.pl>, the timer removal in
gen_kill_estimator races with the timer function rearming the timer.

Check whether the timer list is empty before rearming the timer
in the timer function to fix this.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Jarek Poplawski <jarkao2@o2.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/gen_estimator.c

@@ -128,7 +128,8 @@ static void est_timer(unsigned long arg)
 		spin_unlock(e->stats_lock);
 	}
 
-	mod_timer(&elist[idx].timer, jiffies + ((HZ<<idx)/4));
+	if (elist[idx].list != NULL)
+		mod_timer(&elist[idx].timer, jiffies + ((HZ<<idx)/4));
 	read_unlock(&est_lock);
 }