remoteproc: imx_dsp_rproc: Add mutex protection for workqueue

In the Linux kernel, the following vulnerability has been resolved:

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.