br full backup fail during flashback for dead lock

[seiya-annie]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. run workload
2. run flashback to before workload run
3. br full backup during flashback
4. br restore to dest tidb from minio files
5. check dest tidb crc32 checksum

flashback cluster to timestamp '2023-01-09 09:00';
tiup br backup full --pd [172.16.6.46:2379](http://172.16.6.46:2379/) --backupts '2023-01-09 10:48:00' --storage "s3://tmp/backup-46-snapshot-20230109104800-empty?access-key=minioadmin&secret-access-key=minioadmin&endpoint=http%3a%2f%[2fminio.pingcap.net](http://2fminio.pingcap.net/)%3a9001&force-path-style=true" --ratelimit 128 --log-file backupfull-46-0109.log

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

backup success

3. What did you see instead (Required)

backup hang

4. What is your TiDB version? (Required)

master