pppoe: drop pppoe device in pppoe_unbind_sock_work

After receiving a PADT and the socket is closed, user space will no
longer drop the reference to the pppoe device.
This leads to errors like this:

[  488.570000] unregister_netdevice: waiting for eth0.2 to become free. Usage count = 2

Fixes: 287f3a94 ("pppoe: Use workqueue to die properly when a PADT is received")
Signed-off-by: NFelix Fietkau <nbd@openwrt.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ppp/pppoe.c

@@ -465,6 +465,10 @@ static void pppoe_unbind_sock_work(struct work_struct *work)
 	struct sock *sk = sk_pppox(po);
 
 	lock_sock(sk);
+	if (po->pppoe_dev) {
+		dev_put(po->pppoe_dev);
+		po->pppoe_dev = NULL;
+	}
 	pppox_unbind_sock(sk);
 	release_sock(sk);
 	sock_put(sk);