unit: replace recursive_stop by stop_retroactively to simplify things a little

src/mount.c

@@ -70,6 +70,9 @@ static void mount_init(Unit *u) {
          * already trying to comply its last one. */
         m->exec_context.same_pgrp = true;
 
+        /* Make sure we unmount when the devices we require go away */
+        m->meta.stop_retroactively = true;
+
         m->timer_watch.type = WATCH_INVALID;
 
         m->control_command_id = _MOUNT_EXEC_COMMAND_INVALID;