rtnetlink: allow rtnl_fill_link_netnsid() to run under RCU protection

We want to be able to run rtnl_fill_ifinfo() under RCU protection
instead of RTNL in the future.

All rtnl_link_ops->get_link_net() methods already using dev_net()
are ready. I added READ_ONCE() annotations on others.

Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/ppp/ppp_generic.c

···
 {
 	struct ppp *ppp = netdev_priv(dev);
 
-	return ppp->ppp_net;
+	return READ_ONCE(ppp->ppp_net);
 }
 
 static struct rtnl_link_ops ppp_link_ops __read_mostly = {

drivers/net/vxlan/vxlan_core.c

···
 {
 	struct vxlan_dev *vxlan = netdev_priv(dev);
 
-	return vxlan->net;
+	return READ_ONCE(vxlan->net);
 }
 
 static struct rtnl_link_ops vxlan_link_ops __read_mostly = {

net/core/rtnetlink.c

···
 			goto nla_put_failure;
 	}
 
-	if (rtnl_fill_link_netnsid(skb, dev, src_net, gfp))
-		goto nla_put_failure;
-
 	if (new_nsid &&
 	    nla_put_s32(skb, IFLA_NEW_NETNSID, *new_nsid) < 0)
 		goto nla_put_failure;
···
 		goto nla_put_failure;
 
 	rcu_read_lock();
+	if (rtnl_fill_link_netnsid(skb, dev, src_net, GFP_ATOMIC))
+		goto nla_put_failure_rcu;
 	qdisc = rcu_dereference(dev->qdisc);
 	if (qdisc && nla_put_string(skb, IFLA_QDISC, qdisc->ops->id))
 		goto nla_put_failure_rcu;

net/ipv4/ip_tunnel.c

···
 {
 	struct ip_tunnel *tunnel = netdev_priv(dev);
 
-	return tunnel->net;
+	return READ_ONCE(tunnel->net);
 }
 EXPORT_SYMBOL(ip_tunnel_get_link_net);
 

net/ipv6/ip6_tunnel.c

···
 {
 	struct ip6_tnl *tunnel = netdev_priv(dev);
 
-	return tunnel->net;
+	return READ_ONCE(tunnel->net);
 }
 EXPORT_SYMBOL(ip6_tnl_get_link_net);
 

net/xfrm/xfrm_interface_core.c

···
 {
 	struct xfrm_if *xi = netdev_priv(dev);
 
-	return xi->net;
+	return READ_ONCE(xi->net);
 }
 
 static const struct nla_policy xfrmi_policy[IFLA_XFRM_MAX + 1] = {