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: 287f3a943fe ("pppoe: Use workqueue to die properly when a PADT is received")
Signed-off-by: Felix Fietkau <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ppp/pppoe.c

···
 	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);