MDEV-17236 mariabackup incorrectly tries to open ibdata1

		if custom undo tablespace is defined

- In case of multiple undo tablespace, mariabackup have to open system
tablespace to find the list of undo tablespace present in TRX_SYS page.
For opening system tablespace, mariabackup should fetch the file name
from already initialized system tablespace object.

extra/mariabackup/xtrabackup.cc

@@ -3255,7 +3255,7 @@ static dberr_t xb_assign_undo_space_start()
 	}
 
 	snprintf(name + dirnamelen, (sizeof name) - dirnamelen,
-		 "%s", "ibdata1");
+		 "%s", srv_sys_space.first_datafile()->name());
 
 	file = os_file_create(0, name, OS_FILE_OPEN,
 			      OS_FILE_NORMAL, OS_DATA_FILE, true, &ret);

mysql-test/suite/mariabackup/undo_space_id.opt

@@ -1,2 +1,3 @@
 --debug=d,innodb_undo_upgrade
+--innodb_data_file_path=ib_mysql:10M
 --innodb_undo_tablespaces=2