remoteproc: imx_dsp_rproc: Add mutex protection for workqueue

remoteproc: imx_dsp_rproc: Add mutex protection for workqueue

The workqueue may execute late even after remoteproc is stopped or
stopping, some resources (rpmsg device and endpoint) have been
released in rproc_stop_subdevices(), then rproc_vq_interrupt()
accessing these resources will cause kennel dump.

Call trace:
 virtqueue_add_split+0x1ac/0x560
 virtqueue_add_inbuf+0x4c/0x60
 rpmsg_recv_done+0x15c/0x294
 vring_interrupt+0x6c/0xa4
 rproc_vq_interrupt+0x30/0x50
 imx_dsp_rproc_vq_work+0x24/0x40 [imx_dsp_rproc]
 process_one_work+0x1d0/0x354
 worker_thread+0x13c/0x470
 kthread+0x154/0x160
 ret_from_fork+0x10/0x20

Add mutex protection in imx_dsp_rproc_vq_work(), if the state is
not running, then just skip calling rproc_vq_interrupt().

Also the flush workqueue operation can't be added in rproc stop
for the same reason. The call sequence is

rproc_shutdown
-> rproc_stop
   ->rproc_stop_subdevices
   ->rproc->ops->stop()
     ->imx_dsp_rproc_stop
       ->flush_work
         -> rproc_vq_interrupt

The resource needed by rproc_vq_interrupt has been released in
rproc_stop_subdevices, so flush_work is not safe to be called in
imx_dsp_rproc_stop.

Fixes: ec0e5549f358 ("remoteproc: imx_dsp_rproc: Add remoteproc driver for DSP on i.MX")
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/1664524216-19949-1-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>