owner: fix data race on ownerManager.campaignCancel (#56362)

[ti-chi-bot]

This is an automated cherry-pick of #56362

What problem does this PR solve?

Issue Number: close #56053

Problem Summary: m.campaignCancel is currently set in campaignLoop goroutine, which might happen after calling campaignCancel() that access m.campaignCancel, even though it is called after CampaignOwner().

What changed and how does it work?

Set m.campaignCancel in CampaignOwner() rather than in campaignLoop goroutine, to ensure a campaignCancel() called after CampaignOwner() can always access m.campaignCancel instead of nil.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	a584801	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	a584801	link	true	/test check-dev
idc-jenkins-ci-tidb/build	a584801	link	true	/test build
idc-jenkins-ci-tidb/check_dev_2	a584801	link	true	/test check-dev2

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