wrong order of checking fast DDL available and cleanup

[lance6716]

Bug Report

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1. Minimal reproduce step (Required)

tidb/pkg/ddl/index.go

Lines 792 to 798 in 6660de7

	available, err := ingest.LitBackCtxMgr.CheckAvailable()
	if err != nil {
	return model.ReorgTypeNone, err
	}
	if available {
	ctx := logutil.WithCategory(ctx, "ddl-ingest")
	err = cleanupSortPath(ctx, job.ID)

if the free space is less than 10% we will always fail on CheckAvailable, no chance to run cleanupSortPath

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)