flush kacpi_notify_wq before removing notify handler

Flush kacpi_notify_wq before notify handler is removed,
this can fix a bug which the deferred notify handler is executed
after the notify_handler has already been removed.
http://bugzilla.kernel.org/show_bug.cgi?id=9772

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/osl.c

@@ -742,6 +742,7 @@ EXPORT_SYMBOL(acpi_os_execute);
 void acpi_os_wait_events_complete(void *context)
 {
 	flush_workqueue(kacpid_wq);
+	flush_workqueue(kacpi_notify_wq);
 }
 
 EXPORT_SYMBOL(acpi_os_wait_events_complete);