ipvlan: hold lower dev to avoid possible use-after-free

Recently syzkaller discovered the issue of disappearing lower
device (NETDEV_UNREGISTER) while the virtual device (like
macvlan) is still having it as a lower device. So it's just
a matter of time similar discovery will be made for IPvlan
device setup. So fixing it preemptively. Also while at it,
add a refcount tracker.

Fixes: 2ad7bf363841 ("ipvlan: Initial check-in of the IPVLAN driver.")
Signed-off-by: Mahesh Bandewar <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ipvlan/ipvlan.h

···
 	struct sk_buff_head	backlog;
 	int			count;
 	struct ida		ida;
+	netdevice_tracker	dev_tracker;
 };
 
 struct ipvl_skb_cb {

drivers/net/ipvlan/ipvlan_main.c

···
 	if (err)
 		goto err;
 
+	netdev_hold(dev, &port->dev_tracker, GFP_KERNEL);
 	return 0;
 
 err:
···
 	struct ipvl_port *port = ipvlan_port_get_rtnl(dev);
 	struct sk_buff *skb;
 
+	netdev_put(dev, &port->dev_tracker);
 	if (port->mode == IPVLAN_MODE_L3S)
 		ipvlan_l3s_unregister(port);
 	netdev_rx_handler_unregister(dev);