add index and import into may generate wrong plan with more nodes

[ywqzzy]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. deploy 2 tidb with label "background" and "".
2. set global tidb_enable_dist_task=on;
3. add index

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

Add index run on tidb with label “background" with subtask cnt cnt1.

3. What did you see instead (Required)

Add index run on tidb with label “background" with subtask cnt cnt2 = 2*cnt1.

4. What is your TiDB version? (Required)

master