mcs: fix prepare checker panic

[rleungx]

What problem does this PR solve?

Issue Number: Close #7735

What is changed and how does it work?

When one PD becomes the leader again, the coordinator won't be created because of the misuse of once.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• CabinfeverB
• lhy1024

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[rleungx]

/merge

[ti-chi-bot]

@rleungx: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 313f7eb