[NET_SCHED] Do not oops when user tries to attach a filter to a TBF qdisc.

(TBF does not allow attaching filters as it has only one class,
filter should be attached either to TBF's parent or to its child)

net/sched/sch_tbf.c

@@ -495,6 +495,11 @@ static void tbf_walk(struct Qdisc *sch, struct qdisc_walker *walker)
 	}
 }
 
+static struct tcf_proto **tbf_find_tcf(struct Qdisc *sch, unsigned long cl)
+{
+	return NULL;
+}
+
 static struct Qdisc_class_ops tbf_class_ops =
 {
 	.graft		=	tbf_graft,
@@ -504,6 +509,7 @@ static struct Qdisc_class_ops tbf_class_ops =
 	.change		=	tbf_change_class,
 	.delete		=	tbf_delete,
 	.walk		=	tbf_walk,
+	.tcf_chain	=	tbf_find_tcf,
 	.dump		=	tbf_dump_class,
 };