[media] v4l: Allow calling v4l2_device_register_subdev_nodes() multiple times

Previously multiple calls to v4l2_device_register_subdev_nodes() ended up
corrupting memory and leaking some, too. This patch changes the behaviour so
that sub-devices the device nodes of which are already registered are
ignored.

Signed-off-by: Sebastian Reichel <[e-mail]>
Signed-off-by: Ivaylo Dimitrov <[e-mail]>
Signed-off-by: Pavel Machek <[e-mail]>
Signed-off-by: Sakari Ailus <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

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

···
 		if (!(sd->flags & V4L2_SUBDEV_FL_HAS_DEVNODE))
 			continue;
 
+		if (sd->devnode)
+			continue;
+
 		vdev = kzalloc(sizeof(*vdev), GFP_KERNEL);
 		if (!vdev) {
 			err = -ENOMEM;