`ownerManager` has race even when its methods are serialized called

[lance6716]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

call CampaignOwner then CampaignCancel

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

3. What did you see instead (Required)

tidb/pkg/owner/manager.go

Line 199 in d6db1e8

go m.campaignLoop(session)

tidb/pkg/owner/manager.go

Line 246 in d6db1e8

campaignContext, m.campaignCancel = context.WithCancel(m.ctx)

tidb/pkg/owner/manager.go

Lines 238 to 240 in d6db1e8

	// CampaignCancel implements Manager.CampaignCancel interface.
	func (m *ownerManager) CampaignCancel() {
	m.campaignCancel()

there's data race or panic on campaignCancel

4. What is your TiDB version? (Required)

[lance6716]

ptal @fishiu