DATA RACE in the github.com/tikv/client-go/v2/internal/unionstore.(*memdbArena)

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

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

### 3. What did you see instead (Required)

```
==================
WARNING: DATA RACE
Read at 0x00c015035060 by goroutine 53915:
github.com/tikv/client-go/v2/internal/unionstore.(*nodeAllocator).getNode()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb_arena.go:251 +0x96
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).getNode()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:779 +0x29
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).getRoot()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:783 +0x87
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).traverse()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:389 +0x88
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).Get()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:209 +0x98
github.com/pingcap/tidb/pkg/store/driver/txn.(*memBuffer).Get()
pkg/store/driver/txn/unionstore_driver.go:60 +0x5d
github.com/pingcap/tidb/pkg/executor.(*PointGetExecutor).get()
pkg/executor/point_get.go:463 +0x155
github.com/pingcap/tidb/pkg/executor.(*PointGetExecutor).getAndLock()
pkg/executor/point_get.go:373 +0x2c4
github.com/pingcap/tidb/pkg/executor.(*PointGetExecutor).Next()
pkg/executor/point_get.go:310 +0xe28
github.com/pingcap/tidb/pkg/executor/internal/exec.Next()
pkg/executor/internal/exec/executor.go:309 +0x34b
github.com/pingcap/tidb/pkg/executor.(*SelectLockExec).Next()
pkg/executor/executor.go:1160 +0xe5
github.com/pingcap/tidb/pkg/executor/internal/exec.Next()
pkg/executor/internal/exec/executor.go:309 +0x34b
github.com/pingcap/tidb/pkg/executor.(*UnionExec).resultPuller()
pkg/executor/executor.go:1883 +0x989
github.com/pingcap/tidb/pkg/executor.(*UnionExec).initialize.func1()
pkg/executor/executor.go:1836 +0x5d
Previous write at 0x00c015035060 by goroutine 53916:
github.com/tikv/client-go/v2/internal/unionstore.(*memdbArena).enlarge()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb_arena.go:128 +0x206
github.com/tikv/client-go/v2/internal/unionstore.(*memdbArena).alloc()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb_arena.go:107 +0x86
github.com/tikv/client-go/v2/internal/unionstore.(*nodeAllocator).allocNode()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb_arena.go:257 +0x8a
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).allocNode()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:789 +0xf1
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).traverse()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:410 +0x3af
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).set()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:335 +0x1cf
github.com/tikv/client-go/v2/internal/unionstore.(*MemDB).UpdateFlags()
external/com_github_tikv_client_go_v2/internal/unionstore/memdb.go:250 +0x2aae
github.com/tikv/client-go/v2/txnkv/transaction.(*KVTxn).lockKeys()
external/com_github_tikv_client_go_v2/txnkv/transaction/txn.go:1242 +0x29ed
github.com/tikv/client-go/v2/txnkv/transaction.(*KVTxn).LockKeysFunc()
external/com_github_tikv_client_go_v2/txnkv/transaction/txn.go:919 +0x11d
github.com/pingcap/tidb/pkg/store/driver/txn.(*tikvTxn).LockKeysFunc()
pkg/store/driver/txn/txn_driver.go:91 +0xb2
github.com/pingcap/tidb/pkg/session.(*LazyTxn).LockKeysFunc()
pkg/session/txn.go:483 +0x38d
github.com/pingcap/tidb/pkg/session.(*LazyTxn).LockKeys()
pkg/session/txn.go:458 +0x84
github.com/pingcap/tidb/pkg/executor.doLockKeys()
pkg/executor/executor.go:1279 +0x30c
github.com/pingcap/tidb/pkg/executor.(*SelectLockExec).Next()
pkg/executor/executor.go:1211 +0x7ae
github.com/pingcap/tidb/pkg/executor/internal/exec.Next()
pkg/executor/internal/exec/executor.go:309 +0x34b
github.com/pingcap/tidb/pkg/executor.(*UnionExec).resultPuller()
pkg/executor/executor.go:1883 +0x989
github.com/pingcap/tidb/pkg/executor.(*UnionExec).initialize.func1()
pkg/executor/executor.go:1836 +0x5d
Goroutine 53915 (running) created at:
github.com/pingcap/tidb/pkg/executor.(*UnionExec).initialize()
pkg/executor/executor.go:1836 +0x517
github.com/pingcap/tidb/pkg/executor.(*UnionExec).Next()
pkg/executor/executor.go:1909 +0xba
github.com/pingcap/tidb/pkg/executor/internal/exec.Next()
pkg/executor/internal/exec/executor.go:309 +0x34b
github.com/pingcap/tidb/pkg/executor/aggregate.(*HashAggExec).fetchChildData()
pkg/executor/aggregate/agg_hash_executor.go:385 +0x338
github.com/pingcap/tidb/pkg/executor/aggregate.(*HashAggExec).prepare4ParallelExec.func3()
pkg/executor/aggregate/agg_hash_executor.go:422 +0x5d
Goroutine 53916 (finished) created at:
github.com/pingcap/tidb/pkg/executor.(*UnionExec).initialize()
pkg/executor/executor.go:1836 +0x517
github.com/pingcap/tidb/pkg/executor.(*UnionExec).Next()
pkg/executor/executor.go:1909 +0xba
github.com/pingcap/tidb/pkg/executor/internal/exec.Next()
pkg/executor/internal/exec/executor.go:309 +0x34b
github.com/pingcap/tidb/pkg/executor/aggregate.(*HashAggExec).fetchChildData()
pkg/executor/aggregate/agg_hash_executor.go:385 +0x338
github.com/pingcap/tidb/pkg/executor/aggregate.(*HashAggExec).prepare4ParallelExec.func3()
pkg/executor/aggregate/agg_hash_executor.go:422 +0x5d
==================
```

### 4. What is your TiDB version? (Required)