pkt_sched: fix warning in net/sched/sch_hfsc.c

this warning:

  net/sched/sch_hfsc.c: In function ‘hfsc_enqueue’:
  net/sched/sch_hfsc.c:1577: warning: ‘err’ may be used uninitialized in this function

triggers because GCC does not recognize the (correct) error flow
between hfsc_classify(), 'cl' and 'err'.

Annotate it.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_hfsc.c

@@ -1563,7 +1563,7 @@ static int
 hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 {
 	struct hfsc_class *cl;
-	int err;
+	int uninitialized_var(err);
 
 	cl = hfsc_classify(skb, sch, &err);
 	if (cl == NULL) {