MDEV-12814 mariabackup : don't try io throttling in copy-back

Throttling only works with when creating backup.  Attempt to use it with
 --copy-back results in crash, since throttle events are not initialized.
Thus, ignore throttling unless --backup is given.

extra/mariabackup/xtrabackup.cc

@@ -2169,7 +2169,7 @@ xb_write_delta_metadata(const char *filename, const xb_delta_info_t *info)
 void
 xtrabackup_io_throttling(void)
 {
-	if (xtrabackup_throttle && (io_ticket--) < 0) {
+	if (xtrabackup_backup && xtrabackup_throttle && (io_ticket--) < 0) {
 		os_event_reset(wait_throttle);
 		os_event_wait(wait_throttle);
 	}

mysql-test/suite/mariabackup/include/restart_and_restore.inc

@@ -7,7 +7,7 @@ shutdown_server;
 echo # remove datadir;
 rmdir $_datadir;
 echo # xtrabackup move back;
-exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --copy-back --datadir=$_datadir --target-dir=$targetdir --parallel=2;
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --copy-back --datadir=$_datadir --target-dir=$targetdir --parallel=2 --throttle=1;
 echo # restart server;
 exec echo "restart" > $_expect_file_name;
 enable_reconnect;