MDEV-31256 fil_node_open_file() releases fil_system.mutex allowing other thread to open its file node

There is room between mutex_exit(&fil_system.mutex) and
mutex_enter(&fil_system.mutex) calls in fil_node_open_file(). During this
room another thread can open the node, and ut_ad(!node->is_open())
assertion in fil_node_open_file_low() can fail.

The fix is not to open node if it was already opened by another thread.

storage/innobase/fil/fil0fil.cc

@@ -458,7 +458,9 @@ static bool fil_node_open_file(fil_node_t *node)
     }
   }
 
-  return fil_node_open_file_low(node);
+  /* The node can be opened beween releasing and acquiring fil_system.mutex
+  in the above code */
+  return node->is_open() || fil_node_open_file_low(node);
 }
 
 /** Close the file handle. */