net: add netdev_for_each_upper_dev_rcu()

The new macro netdev_for_each_upper_dev_rcu(dev, upper, iter) iterates
through the dev->upper_dev_list starting from the first element, using
the netdev_upper_get_next_dev_rcu(dev, &iter).

Must be called under RCU read lock.

CC: "David S. Miller" <davem@davemloft.net>
CC: Eric Dumazet <edumazet@google.com>
CC: Jiri Pirko <jiri@resnulli.us>
CC: Alexander Duyck <alexander.h.duyck@intel.com>
CC: Cong Wang <amwang@redhat.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2768,6 +2768,16 @@ extern int		bpf_jit_enable;
 extern bool netdev_has_upper_dev(struct net_device *dev,
 				 struct net_device *upper_dev);
 extern bool netdev_has_any_upper_dev(struct net_device *dev);
+extern struct net_device *netdev_upper_get_next_dev_rcu(struct net_device *dev,
+							struct list_head **iter);
+
+/* iterate through upper list, must be called under RCU read lock */
+#define netdev_for_each_upper_dev_rcu(dev, upper, iter) \
+	for (iter = &(dev)->upper_dev_list, \
+	     upper = netdev_upper_get_next_dev_rcu(dev, &(iter)); \
+	     upper; \
+	     upper = netdev_upper_get_next_dev_rcu(dev, &(iter)))
+
 extern struct net_device *netdev_master_upper_dev_get(struct net_device *dev);
 extern struct net_device *netdev_master_upper_dev_get_rcu(struct net_device *dev);
 extern int netdev_upper_dev_link(struct net_device *dev,