uvcvideo: Fix race-related crash in uvc_video_clock_update()

uvcvideo: Fix race-related crash in uvc_video_clock_update()

commit ed0ee0ce0a3224dab5caa088a5f8b6df25924276 upstream.

The driver frees the clock samples buffer before stopping the video
buffers queue. If a DQBUF call arrives in-between,
uvc_video_clock_update() will be called with a NULL clock samples
buffer, leading to a crash. This occurs very frequently when using the
webcam with the flash browser plugin.

Move clock initialization/cleanup to uvc_video_enable() in order to free
the clock samples buffer after the queue is stopped. Make sure the clock
is reset at resume time to avoid miscalculating timestamps.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>