ACPI / SBS: Fix rare oops when removing modules

There was a small race when removing the sbshc module where
smbus_alarm() had queued acpi_smbus_callback() for deferred execution
but it hadn't been run yet, so that when it did run hc had been freed
and the module unloaded, resulting in an invalid paging request.

A similar race existed when removing the sbs module with regards to
acpi_sbs_callback() (which is called from acpi_smbus_callback()).

We therefore need to ensure no callbacks are pending or executing before
the cleanups are done and the modules are removed.

Signed-off-by: Ronald Tschalär <ronald@innovation.ch>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/osl.c

@@ -1129,6 +1129,7 @@ void acpi_os_wait_events_complete(void)
 	flush_workqueue(kacpid_wq);
 	flush_workqueue(kacpi_notify_wq);
 }
+EXPORT_SYMBOL(acpi_os_wait_events_complete);
 
 struct acpi_hp_work {
 	struct work_struct work;