media: v4l2-subdev: handle module refcounting here

The module ownership refcounting was done in media_entity_get/put,
but that was very confusing and it did not work either in case an
application had a v4l-subdevX device open and the module was
unbound. When the v4l-subdevX device was closed the media_entity_put
was never called and the module refcount was left one too high, making
it impossible to unload it.

Since v4l2-subdev.c was the only place where media_entity_get/put was
called, just move the functionality to v4l2-subdev.c and drop those
confusing entity functions.

Store the module in subdev_fh so module_put no longer depends on
the media_entity struct.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

include/media/v4l2-subdev.h

@@ -910,9 +910,11 @@ struct v4l2_subdev {
  *
  * @vfh: pointer to &struct v4l2_fh
  * @pad: pointer to &struct v4l2_subdev_pad_config
+ * @owner: module pointer to the owner of this file handle
  */
 struct v4l2_subdev_fh {
 	struct v4l2_fh vfh;
+	struct module *owner;
 #if defined(CONFIG_VIDEO_V4L2_SUBDEV_API)
 	struct v4l2_subdev_pad_config *pad;
 #endif