branch-2.1: [improve](routine load) ensure load balance after scaling up BE nodes #44693 (#44799)

Cherry-picked from #44693

Co-authored-by: hui lai <laihui@selectdb.com>

fe/fe-core/src/main/java/org/apache/doris/load/routineload/RoutineLoadManager.java

@@ -486,6 +486,7 @@ public class RoutineLoadManager implements Writable {
         // check if be has idle slot
         readLock();
         try {
+            updateBeIdToMaxConcurrentTasks();
             Map<Long, Integer> beIdToConcurrentTasks = getBeCurrentTasksNumMap();
             int previousBeIdleTaskNum = 0;