tidb_analyze_partition_concurrency can too many woker when tidb have less task.

[hawkingrei]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

tidb_analyze_partition_concurrency cannot be used directly. we should check the number of tasks.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)