dcb: Add stub routines for !CONFIG_DCB

To avoid ifdefs in the other code that supports DCB notifiers
add stub routines. This method seems popular in other net code
for example 8021Q.
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dcbevent.h

@@ -24,8 +24,26 @@ enum dcbevent_notif_type {
 	DCB_APP_EVENT = 1,
 };
 
+#ifdef CONFIG_DCB
 extern int register_dcbevent_notifier(struct notifier_block *nb);
 extern int unregister_dcbevent_notifier(struct notifier_block *nb);
 extern int call_dcbevent_notifiers(unsigned long val, void *v);
+#else
+static inline int
+register_dcbevent_notifier(struct notifier_block *nb)
+{
+	return 0;
+}
+
+static inline int unregister_dcbevent_notifier(struct notifier_block *nb)
+{
+	return 0;
+}
+
+static inline int call_dcbevent_notifiers(unsigned long val, void *v)
+{
+	return 0;
+}
+#endif /* CONFIG_DCB */
 
 #endif