media: pwc: Drain fill_buf on start_streaming() failure

media: pwc: Drain fill_buf on start_streaming() failure

pwc_isoc_init() submits its isochronous URBs with
usb_submit_urb(.., GFP_KERNEL) in a loop. After the first URB is
submitted, its completion handler pwc_isoc_handler() can run on another
CPU before the loop finishes:

  start_streaming()
    pwc_isoc_init()
      usb_submit_urb(urbs[0], GFP_KERNEL)
                                  pwc_isoc_handler(urbs[0])
                                    pdev->fill_buf =
                                      pwc_get_next_fill_buf(pdev)
      usb_submit_urb(urbs[i>0], ..)  -> fails
      pwc_isoc_cleanup(pdev)           /* kills URBs */
      return ret;
    pwc_cleanup_queued_bufs(pdev, VB2_BUF_STATE_QUEUED)

pwc_get_next_fill_buf() detaches a buffer from pdev->queued_bufs and
stores it in pdev->fill_buf. The error path in start_streaming() only
drains pdev->queued_bufs, so the buffer parked in pdev->fill_buf is
leaked. vb2_start_streaming() then triggers
WARN_ON(owned_by_drv_count).

stop_streaming() already handles this since commit 80b0963e1698
("[media] pwc: fix WARN_ON"), which added the fill_buf drain in the
teardown path but not in the start_streaming() error path. Mirror that
handling on failure so start_streaming() returns with no buffer owned
by the driver.

Issue identified by automated review of the INV-003 series at
https://sashiko.dev/

Fixes: 885fe18f5542 ("[media] pwc: Replace private buffer management code with videobuf2")
Cc: stable@vger.kernel.org
Signed-off-by: Valery Borovsky <vebohr@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>