branch-2.1:[fix](cooldown)No need to rdlock inside get_cooldown_tablets, there's enough rdlock inside tablet internal function calls.#39211

[dtkavin]

cherry-pick: #39211

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[dtkavin]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	39.03% (10326/26457)
Line Coverage	30.03% (85770/285652)
Region Coverage	28.69% (44259/154270)
Branch Coverage	25.42% (22652/89126)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	74.58% (19435/26061)
Line Coverage	69.24% (196515/283808)
Region Coverage	66.83% (120003/179559)
Branch Coverage	59.87% (60328/100758)

[dataroaring]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.