cpuidle: move driver checking within the lock section

The code is racy and the check with cpuidle_curr_driver should be
done under the lock.

I don't find a path in the different drivers where that could happen
because the arch specific drivers are written in such way it is not
possible to register a driver while it is unregistered, except maybe
in a very improbable case when "intel_idle" and "processor_idle" are
competing. One could unregister a driver, while the other one is
registering.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpuidle/driver.c

@@ -85,17 +85,9 @@ EXPORT_SYMBOL_GPL(cpuidle_get_driver);
  */
 void cpuidle_unregister_driver(struct cpuidle_driver *drv)
 {
-	if (drv != cpuidle_curr_driver) {
-		WARN(1, "invalid cpuidle_unregister_driver(%s)\n",
-			drv->name);
-		return;
-	}
-
 	spin_lock(&cpuidle_driver_lock);
-
-	if (!WARN_ON(drv->refcnt > 0))
+	if (drv == cpuidle_curr_driver && !WARN_ON(drv->refcnt > 0))
 		cpuidle_curr_driver = NULL;
-
 	spin_unlock(&cpuidle_driver_lock);
 }
 EXPORT_SYMBOL_GPL(cpuidle_unregister_driver);