vlan: use xarray iterator to implement /proc/net/vlan/config

Adopt net->dev_by_index as I did in commit 0e0939c0
("net-procfs: use xarray iterator to implement /proc/net/dev")

Not only this removes quadratic behavior, it also makes sure
an existing vlan device is always visible in the dump,
regardless of concurrent net->dev_base_head changes.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240211214404.1882191-2-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/8021q/vlanproc.c

@@ -163,48 +163,34 @@ void vlan_proc_rem_dev(struct net_device *vlandev)
  * The following few functions build the content of /proc/net/vlan/config
  */
 
-/* start read of /proc/net/vlan/config */
-static void *vlan_seq_start(struct seq_file *seq, loff_t *pos)
-	__acquires(rcu)
+static void *vlan_seq_from_index(struct seq_file *seq, loff_t *pos)
 {
+	unsigned long ifindex = *pos;
 	struct net_device *dev;
-	struct net *net = seq_file_net(seq);
-	loff_t i = 1;
-
-	rcu_read_lock();
-	if (*pos == 0)
-		return SEQ_START_TOKEN;
 
-	for_each_netdev_rcu(net, dev) {
+	for_each_netdev_dump(seq_file_net(seq), dev, ifindex) {
 		if (!is_vlan_dev(dev))
 			continue;
-
-		if (i++ == *pos)
-			return dev;
+		*pos = dev->ifindex;
+		return dev;
 	}
+	return NULL;
+}
+
+static void *vlan_seq_start(struct seq_file *seq, loff_t *pos)
+	__acquires(rcu)
+{
+	rcu_read_lock();
+	if (*pos == 0)
+		return SEQ_START_TOKEN;
 
-	return  NULL;
+	return vlan_seq_from_index(seq, pos);
 }
 
 static void *vlan_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 {
-	struct net_device *dev;
-	struct net *net = seq_file_net(seq);
-
 	++*pos;
-
-	dev = v;
-	if (v == SEQ_START_TOKEN)
-		dev = net_device_entry(&net->dev_base_head);
-
-	for_each_netdev_continue_rcu(net, dev) {
-		if (!is_vlan_dev(dev))
-			continue;
-
-		return dev;
-	}
-
-	return NULL;
+	return vlan_seq_from_index(seq, pos);
 }
 
 static void vlan_seq_stop(struct seq_file *seq, void *v)