add index using local sort in DXF should be optimize for small table

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

For a small table, we should not split the add index task into too many subtasks.
These tasks' range overlaps and may cause the "too many SST files" problem.

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

N/A

3. What did you see instead (Required)

N/A

4. What is your TiDB version? (Required)

master, v8.5