[Bug] Clear Txn when load been cancelled (apache#3766)

If you a load task encoutering error, it will be cancelled.
At this time, FE will clear the Txn according to the DbName.
In FE, DbName should be added by cluter name.
If missing cluster name, it will encounter NullPointer.
As a result, the Txn will still exists until timeout.

fe/src/main/java/org/apache/doris/service/FrontendServiceImpl.java

@@ -808,7 +808,8 @@ private void loadTxnRollbackImpl(TLoadTxnRollbackRequest request) throws UserExc
             checkPasswordAndPrivs(cluster, request.getUser(), request.getPasswd(), request.getDb(),
                     request.getTbl(), request.getUser_ip(), PrivPredicate.LOAD);
         }
-        Database db = Catalog.getInstance().getDb(request.getDb());
+        String dbName = ClusterNamespace.getFullName(cluster, request.getDb());
+        Database db = Catalog.getInstance().getDb(dbName);
         if (db == null) {
             throw new MetaNotFoundException("db " + request.getDb() + " does not exist");
         }