Buffers were marked as done before being removed from the IRQ queue. If
a userspace application dequeued and requeued the buffer fast enough
during that time window, the buffer could end up being deleted twice,
generating an oops in interrupt context.

Add a new state, UVC_BUF_STATE_READY, to mark buffers as ready for reuse
but not yet removed from the queue, and transition to UVC_BUF_STATE_DONE
only when the buffer is removed from the queue.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>