PS/2 kb allow only one suspended proc at a time

kernel/device/ps2_kb.c

@@ -215,8 +215,18 @@ int ps2kb_read_key (struct device* device, struct device_op_ctx* op_ctx, void* a
 
   /* didn't pop anything */
   if (prev_count == new_count) {
+    spin_lock (&ps2kb_sq.lock);
+    struct list_node_link* node = ps2kb_sq.proc_list;
+    spin_unlock (&ps2kb_sq.lock);
+
+    if (node != NULL) {
+      spin_unlock (&ps2kb_ringbuffer_lock);
+      return -ST_PERMISSION_ERROR;
+    }
+
     *op_ctx->reschedule =
         proc_sq_suspend (op_ctx->proc, &ps2kb_sq, &ps2kb_ringbuffer_lock, op_ctx->reschedule_cpu);
+
     return ST_OK;
   }