Tearing down a UVC gadget that userspace still has open corrupts the V4L2 core:
the node number leaks (the gadget climbs /dev/video0 -> video1 -> video2 across
role switches), and once the kernel is tainted by the WARN that goes with it, a
NULL dereference follows with v4l2_device_release on the backtrace.
struct uvc_device embeds its struct video_device and uvc_free() kfree()s the
lot, but the V4L2 core holds a reference to that video_device for as long as
/dev/videoN is open. Reference-count the structure instead, which is what the
driver's own "TODO reference counting" has been asking for.
Verified on the bench: the gadget returns to /dev/video0 every time instead of
climbing, and no WARN or Oops after repeated switching.
