mount-util: beef up bind_remount_recursive() to be able to toggle more than MS_RDONLY

The function is otherwise generic enough to toggle other bind mount
flags beyond MS_RDONLY (for example: MS_NOSUID or MS_NODEV), hence let's
beef it up slightly to support that too.

src/nspawn/nspawn.c

@@ -3323,7 +3323,7 @@ static int outer_child(
                 return r;
 
         if (arg_read_only && arg_volatile_mode == VOLATILE_NO) {
-                r = bind_remount_recursive(directory, true, NULL);
+                r = bind_remount_recursive(directory, MS_RDONLY, MS_RDONLY, NULL);
                 if (r < 0)
                         return log_error_errno(r, "Failed to make tree read-only: %m");
         }