PM: Allow pm_runtime_suspend() to succeed during system suspend

The dpm_prepare() function increments the runtime PM reference
counters of all devices to prevent pm_runtime_suspend() from
executing subsystem-level callbacks.  However, this was supposed to
guard against a specific race condition that cannot happen, because
the power management workqueue is freezable, so pm_runtime_suspend()
can only be called synchronously during system suspend and we can
rely on subsystems and device drivers to avoid doing that
unnecessarily.

Make dpm_prepare() drop the runtime PM reference to each device
after making sure that runtime resume is not pending for it.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Kevin Hilman <khilman@ti.com>

drivers/base/power/main.c

@@ -669,7 +669,6 @@ static void dpm_complete(pm_message_t state)
 		mutex_unlock(&dpm_list_mtx);
 
 		device_complete(dev, state);
-		pm_runtime_put_sync(dev);
 
 		mutex_lock(&dpm_list_mtx);
 		put_device(dev);
@@ -1005,12 +1004,9 @@ static int dpm_prepare(pm_message_t state)
 		if (pm_runtime_barrier(dev) && device_may_wakeup(dev))
 			pm_wakeup_event(dev, 0);
 
-		if (pm_wakeup_pending()) {
-			pm_runtime_put_sync(dev);
-			error = -EBUSY;
-		} else {
-			error = device_prepare(dev, state);
-		}
+		pm_runtime_put_sync(dev);
+		error = pm_wakeup_pending() ?
+				-EBUSY : device_prepare(dev, state);
 
 		mutex_lock(&dpm_list_mtx);
 		if (error) {