xen-netback: cancel the credit timer when taking the vif down

If the credit timer is left armed after calling
xen_netbk_remove_xenvif(), then it may fire and attempt to schedule
the vif which will then oops as vif->netbk == NULL.

This may happen both in the fatal error path and during normal
disconnection from the front end.

The sequencing during shutdown is critical to ensure that: a)
vif->netbk doesn't become unexpectedly NULL; and b) the net device/vif
is not freed.

1. Mark as unschedulable (netif_carrier_off()).
2. Synchronously cancel the timer.
3. Remove the vif from the schedule list.
4. Remove it from it netback thread group.
5. Wait for vif->refcnt to become 0.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reported-by: Christopher S. Aker <caker@theshore.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/interface.c

@@ -132,6 +132,7 @@ static void xenvif_up(struct xenvif *vif)
 static void xenvif_down(struct xenvif *vif)
 {
 	disable_irq(vif->irq);
+	del_timer_sync(&vif->credit_timeout);
 	xen_netbk_deschedule_xenvif(vif);
 	xen_netbk_remove_xenvif(vif);
 }
@@ -363,8 +364,6 @@ void xenvif_disconnect(struct xenvif *vif)
 	atomic_dec(&vif->refcnt);
 	wait_event(vif->waiting_to_free, atomic_read(&vif->refcnt) == 0);
 
-	del_timer_sync(&vif->credit_timeout);
-
 	if (vif->irq)
 		unbind_from_irqhandler(vif->irq, vif);