xen-netfront: Fix hang on device removal

A toolstack may delete the vif frontend and backend xenstore entries
while xen-netfront is in the removal code path.  In that case, the
checks for xenbus_read_driver_state would return XenbusStateUnknown, and
xennet_remove would hang indefinitely.  This hang prevents system
shutdown.

xennet_remove must be able to handle XenbusStateUnknown, and
netback_changed must also wake up the wake_queue for that state as well.

Fixes: 5b5971df ("xen-netfront: remove warning when unloading module")
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Cc: Eduardo Otubo <otubo@redhat.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/net/xen-netfront.c

@@ -2005,7 +2005,10 @@ static void netback_changed(struct xenbus_device *dev,
 	case XenbusStateInitialised:
 	case XenbusStateReconfiguring:
 	case XenbusStateReconfigured:
+		break;
+
 	case XenbusStateUnknown:
+		wake_up_all(&module_unload_q);
 		break;
 
 	case XenbusStateInitWait:
@@ -2136,7 +2139,9 @@ static int xennet_remove(struct xenbus_device *dev)
 		xenbus_switch_state(dev, XenbusStateClosing);
 		wait_event(module_unload_q,
 			   xenbus_read_driver_state(dev->otherend) ==
-			   XenbusStateClosing);
+			   XenbusStateClosing ||
+			   xenbus_read_driver_state(dev->otherend) ==
+			   XenbusStateUnknown);
 
 		xenbus_switch_state(dev, XenbusStateClosed);
 		wait_event(module_unload_q,