zdtm.sh: exit with zero in case of success

A bash script exits with code of a last command,
so zdtm.sh return 1 if TMP_TREE is not initialized.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.sh

@@ -469,4 +469,4 @@ else
 	fi
 fi
 
-[ -n "$TMP_TREE" ] && rm -rf $TMP_TREE
+[ -n "$TMP_TREE" ] && rm -rf $TMP_TREE || exit 0