[media] v4l: Make sure we hold a reference to the v4l2_device before using it

v4l2_device_disconnect() calls dev_get_drvdata() and dev_set_drvdata()
on the device it received in v4l2_device_register(). Get a reference to
the device in v4l2_device_register() to make sure it won't disappear as
long as we need it.

Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/v4l2-device.c

@@ -38,6 +38,7 @@ int v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev)
 	mutex_init(&v4l2_dev->ioctl_lock);
 	v4l2_prio_init(&v4l2_dev->prio);
 	kref_init(&v4l2_dev->ref);
+	get_device(dev);
 	v4l2_dev->dev = dev;
 	if (dev == NULL) {
 		/* If dev == NULL, then name must be filled in by the caller */
@@ -93,6 +94,7 @@ void v4l2_device_disconnect(struct v4l2_device *v4l2_dev)
 
 	if (dev_get_drvdata(v4l2_dev->dev) == v4l2_dev)
 		dev_set_drvdata(v4l2_dev->dev, NULL);
+	put_device(v4l2_dev->dev);
 	v4l2_dev->dev = NULL;
 }
 EXPORT_SYMBOL_GPL(v4l2_device_disconnect);