WAN: flush tx_queue in hdlc_ppp to prevent panic on rmmod hw_driver.

tx_queue is used as a temporary queue when not allowed to queue skb
directly to the hw device driver (which may sleep). Most paths flush
it before returning, but ppp_start() currently cannot. Make sure we
don't leave skbs pointing to a non-existent device.

Thanks to Michael Barkowski for reporting this problem.
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wan/hdlc_ppp.c

@@ -628,9 +628,15 @@ static void ppp_stop(struct net_device *dev)
 	ppp_cp_event(dev, PID_LCP, STOP, 0, 0, 0, NULL);
 }
 
+static void ppp_close(struct net_device *dev)
+{
+	ppp_tx_flush();
+}
+
 static struct hdlc_proto proto = {
 	.start		= ppp_start,
 	.stop		= ppp_stop,
+	.close		= ppp_close,
 	.type_trans	= ppp_type_trans,
 	.ioctl		= ppp_ioctl,
 	.netif_rx	= ppp_rx,