[media] v4l2-async: Don't use dynamic static allocation

Dynamic static allocation is evil, as Kernel stack is too low, and
compilation complains about it on some archs:
drivers/media/v4l2-core/v4l2-async.c:238:1: warning: 'v4l2_async_notifier_unregister' uses dynamic stack allocation [enabled by default]
Instead, let's enforce a limit for the buffer.
In this specific case, there's a hard limit imposed by V4L2_MAX_SUBDEVS,
with is currently 128. That means that the buffer size can be up to
128x8 = 1024 bytes (on a 64bits kernel), with is too big for stack.
Worse than that, someone could increase it and cause real troubles.
So, let's use dynamically allocated data, instead.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Reviewed-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/v4l2-core/v4l2-async.c

···
 	struct v4l2_subdev *sd, *tmp;
 	unsigned int notif_n_subdev = notifier->num_subdevs;
 	unsigned int n_subdev = min(notif_n_subdev, V4L2_MAX_SUBDEVS);
-	struct device *dev[n_subdev];
+	struct device **dev;
 	int i = 0;
 
 	if (!notifier->v4l2_dev)
 		return;
+
+	dev = kmalloc(n_subdev * sizeof(*dev), GFP_KERNEL);
+	if (!dev) {
+		dev_err(notifier->v4l2_dev->dev,
+			"Failed to allocate device cache!\n");
+	}
 
 	mutex_lock(&list_lock);
 
 	list_del(&notifier->list);
 
 	list_for_each_entry_safe(sd, tmp, &notifier->done, async_list) {
-		dev[i] = get_device(sd->dev);
+		struct device *d;
+
+		d = get_device(sd->dev);
 
 		v4l2_async_cleanup(sd);
 
 		/* If we handled USB devices, we'd have to lock the parent too */
-		device_release_driver(dev[i++]);
+		device_release_driver(d);
 
 		if (notifier->unbind)
 			notifier->unbind(notifier, sd, sd->asd);
+
+		/*
+		 * Store device at the device cache, in order to call
+		 * put_device() on the final step
+		 */
+		if (dev)
+			dev[i++] = d;
+		else
+			put_device(d);
 	}
 
 	mutex_unlock(&list_lock);
 
+	/*
+	 * Call device_attach() to reprobe devices
+	 *
+	 * NOTE: If dev allocation fails, i is 0, and the whole loop won't be
+	 * executed.
+	 */
 	while (i--) {
 		struct device *d = dev[i];
 
···
 		}
 		put_device(d);
 	}
+	kfree(dev);
 
 	notifier->v4l2_dev = NULL;