V4L2 api can indicate that flushing of the capture buffers is completed
by setting the V4L2_BUF_FLAG_LAST flag.
Use guards because the flag was only defined in Linux v4.2.

Reference:
linux/Documentation/media/uapi/v4l/dev-decoder.rst

    "The client must continue to handle both queues independently,
     similarly to normal decode operation. This includes:
     ...
     - queuing and dequeuing CAPTURE buffers, until a buffer marked with
       the V4L2_BUF_FLAG_LAST flag is dequeued"
Reviewed-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>