media: rkvdec: fix use after free bug in rkvdec_remove

In rkvdec_probe, rkvdec->watchdog_work is bound with
rkvdec_watchdog_func. Then rkvdec_vp9_run may
be called to start the work.

If we remove the module which will call rkvdec_remove
 to make cleanup, there may be a unfinished work.
 The possible sequence is as follows, which will
 cause a typical UAF bug.

Fix it by canceling the work before cleanup in rkvdec_remove.

CPU0                  CPU1

                    |rkvdec_watchdog_func
rkvdec_remove       |
 rkvdec_v4l2_cleanup|
  v4l2_m2m_release  |
    kfree(m2m_dev); |
                    |
                    | v4l2_m2m_get_curr_priv
                    |   m2m_dev->curr_ctx //use

Fixes: cd33c830 ("media: rkvdec: Add the rkvdec driver")
Signed-off-by: Zheng Wang <zyytlz.wz@163.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>