remoteproc: stm32_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 kernel dump.

Call trace:
virtqueue_add_inbuf
virtqueue_add_inbuf
rpmsg_recv_single
rpmsg_recv_done
vring_interrupt
stm32_rproc_mb_vq_work
process_one_work
worker_thread
kthread

Suggested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20230331160634.3113031-1-arnaud.pouliquen@foss.st.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

Author: arnopo

drivers/remoteproc/stm32_rproc.c

@@ -291,8 +291,16 @@ static void stm32_rproc_mb_vq_work(struct work_struct *work)
 	struct stm32_mbox *mb = container_of(work, struct stm32_mbox, vq_work);
 	struct rproc *rproc = dev_get_drvdata(mb->client.dev);
 
+	mutex_lock(&rproc->lock);
+
+	if (rproc->state != RPROC_RUNNING)
+		goto unlock_mutex;
+
 	if (rproc_vq_interrupt(rproc, mb->vq_id) == IRQ_NONE)
 		dev_dbg(&rproc->dev, "no message found in vq%d\n", mb->vq_id);
+
+unlock_mutex:
+	mutex_unlock(&rproc->lock);
 }
 
 static void stm32_rproc_mb_callback(struct mbox_client *cl, void *data)