reorganize partition can not concurrent run ddl with other ddl

[aytrack]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. session 1

create database db1;
CREATE TABLE `t1` (
    `id` bigint(20) NOT NULL AUTO_INCREMENT,
    `k` int(11) NOT NULL DEFAULT '0',
    `c` char(120) NOT NULL DEFAULT '',
    `pad` char(60) NOT NULL DEFAULT '',
    PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */,
    KEY `k_1` (`k`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
  PARTITION BY RANGE (`id`)
  (PARTITION `p0` VALUES LESS THAN (2000000),
   PARTITION `p1` VALUES LESS THAN (4000000),
   PARTITION `p2` VALUES LESS THAN (6000000),
   PARTITION `p3` VALUES LESS THAN (8000000),
   PARTITION `p4` VALUES LESS THAN (10000000),
   PARTITION `p5` VALUES LESS THAN (MAXVALUE));
insert into t1(k, c, pad) values (1, 'a', 'beijing'), (2, 'b', 'chengdu');
insert into t1(k, c, pad) select k, c, pad from t1 limit 2000000; -- some times
alter table t1 reorganize partition p0, p1, p2, p3, p4 into( partition pnew values less than (10000000));

2. session 2

create database db2;
use db2;
create table t01(a int, b int);

2. What did you expect to see? (Required)

session 2 create table t01 will not be blocked by reorganize partition

3. What did you see instead (Required)

create table ddl was blocked by reorganize partition

4. What is your TiDB version? (Required)

partitioninsert_1647963896275> select tidb_version();
+-----------------------------------------------------------+
| tidb_version()                                            |
+-----------------------------------------------------------+
| Release Version: v7.0.0                                   |
| Edition: Community                                        |
| Git Commit Hash: fc166ffae8576ac3efb2f5abac848d5edbaf2657 |
| Git Branch: heads/refs/tags/v7.0.0                        |
| UTC Build Time: 2023-03-20 07:08:59                       |
| GoVersion: go1.20.2                                       |
| Race Enabled: false                                       |
| TiKV Min Version: 6.2.0-alpha                             |
| Check Table Before Drop: false                            |
| Store: tikv                                               |
+-----------------------------------------------------------+

[bb7133]

The root cause is that we didn't mark "reorganize partition" as "need backfill" so it is scheduled as the 'general job', and the 'general job' is scheduled in serial(always block each others).