br: Fine grained backup may fall into infinite retry

[RidRisR]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Runs a backup, when it enter the fine grained stage, if there is a issue that make some tikv disconnect with br, br may fall into infinite retry to reset client.

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

Backup could fast fail and exit.

3. What did you see instead (Required)

It kept retrying.

4. What is your TiDB version? (Required)

v.7.1