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ARROW-13974: [C++]  Resolve follow-up reviews for TopK/BottomK

There are some follow-up reviews for topK/bottomK kernel (ARROW-1565) that were requested after PR was closed. This issue is to resolve them.

Closes #11132 from aocsa/ARROW-13974

Authored-by: Alexander <aocsa.cs@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>