MDEV-22090 Change buffer is not freed after dropping or rebuilding table

When MDEV-19514 removed the change buffer merges in the background,
it introduced a minor regression: After a DDL operation that drops
or rebuilds a table or partition, any entries in the change buffer
could survive until a slow shutdown (innodb_fast_shutdown=0) is executed.

fil_delete_tablespace(). Before returning, invoke
ibuf_delete_for_discarded_space(). With this, the only way to have
change buffer ‘leaks’ is to kill and restart the server at an
appropriate moment and never initiate a slow shutdown.

storage/innobase/fil/fil0fil.cc

@@ -2272,7 +2272,7 @@ dberr_t fil_delete_tablespace(ulint id, bool if_exists)
 				       " in the tablespace memory cache.";
 		}
 
-		return(err);
+		goto func_exit;
 	}
 
 	ut_a(space);
@@ -2367,8 +2367,9 @@ dberr_t fil_delete_tablespace(ulint id, bool if_exists)
 		err = DB_TABLESPACE_NOT_FOUND;
 	}
 
+func_exit:
 	ut_free(path);
-
+	ibuf_delete_for_discarded_space(id);
 	return(err);
 }