netns: net_identifiers should be read_mostly

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/pppol2tp.c b/drivers/net/pppol2tp.c
index 849cc9c..442c382 100644
--- a/drivers/net/pppol2tp.c
+++ b/drivers/net/pppol2tp.c
@@ -232,7 +232,7 @@
 static const struct proto_ops pppol2tp_ops;
 
 /* per-net private data for this module */
-static int pppol2tp_net_id;
+static int pppol2tp_net_id __read_mostly;
 struct pppol2tp_net {
 	struct list_head pppol2tp_tunnel_list;
 	rwlock_t pppol2tp_tunnel_list_lock;