net: add RCU variant to search for netdev_adjacent link

Currently we have only the RTNL flavour, however we can traverse it while
holding only RCU, so add the RCU search. Add an RCU variant that uses
list_head * as an argument, so that it can be universally used afterwards.

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: NVeaceslav Falico <vfalico@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -4380,6 +4380,19 @@ struct netdev_adjacent {
 	struct rcu_head rcu;
 };
 
+static struct netdev_adjacent *__netdev_find_adj_rcu(struct net_device *dev,
+						     struct net_device *adj_dev,
+						     struct list_head *adj_list)
+{
+	struct netdev_adjacent *adj;
+
+	list_for_each_entry_rcu(adj, adj_list, list) {
+		if (adj->dev == adj_dev)
+			return adj;
+	}
+	return NULL;
+}
+
 static struct netdev_adjacent *__netdev_find_adj(struct net_device *dev,
 						 struct net_device *adj_dev,
 						 struct list_head *adj_list)