[bug report] media: dvbdev: adopts refcnt to avoid UAF

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello Lin Ma,

Commit 0fc044b2b5e2 ("media: dvbdev: adopts refcnt to avoid UAF")
from Aug 7, 2022 (linux-next), leads to the following Smatch static
checker warning:

	drivers/media/dvb-core/dvbdev.c:601 dvb_remove_device()
	error: double free of 'dvbdev' (line 598)

drivers/media/dvb-core/dvbdev.c
    591 void dvb_remove_device(struct dvb_device *dvbdev)
    592 {
    593         if (!dvbdev)
    594                 return;
    595 
    596         down_write(&minor_rwsem);
    597         dvb_minors[dvbdev->minor] = NULL;
    598         dvb_device_put(dvbdev);
                ^^^^^^^^^^^^^^^^^^^^^^
If this drops the last reference then it frees "dvbdev" so Smatch complains
about use after frees.

    599         up_write(&minor_rwsem);
    600 
--> 601         dvb_media_device_free(dvbdev);
                                      ^^^^^^

    602 
    603         device_destroy(dvb_class, MKDEV(DVB_MAJOR, dvbdev->minor));
                                                           ^^^^^^^^^^^^^

    604 
    605         list_del(&dvbdev->list_head);
                          ^^^^^^
    606 }

regards,
dan carpenter