driver core: do not wait unnecessarily in driver_unregister()

Ingo reported that built-in drivers suffered bootup hangs with certain
driver unregistry sequences, due to sysfs breakage.

Do the minimal fix for v2.6.21: only wait if the driver is a module.
Acked-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/base/driver.c

@@ -183,7 +183,14 @@ int driver_register(struct device_driver * drv)
 void driver_unregister(struct device_driver * drv)
 {
 	bus_remove_driver(drv);
-	wait_for_completion(&drv->unloaded);
+	/*
+	 * If the driver is a module, we are probably in
+	 * the module unload path, and we want to wait
+	 * for everything to unload before we can actually
+	 * finish the unload.
+	 */
+	if (drv->owner)
+		wait_for_completion(&drv->unloaded);
 }
 
 /**