when only some node hasn't sync, DDL owner always print 'syncer check all versions, all nodes are not synced'

[D3Hunter]

Bug Report

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1. Minimal reproduce step (Required)

because in here, we return false directly on length mismatch, so we don't know who is unmatched

tidb/pkg/ddl/schemaver/syncer.go

Lines 375 to 377 in 042a332

	if len(nodeVersions) < len(updatedMap) {
	return false
	}

and cause this always enter the else branch

tidb/pkg/ddl/schemaver/syncer.go

Lines 397 to 406 in 042a332

	if id := unmatchedNodeID.Load(); id != nil {
	logutil.DDLLogger().Info("syncer check all versions, someone is not synced",
	zap.String("info", *id),
	zap.Int64("ddl job id", jobID),
	zap.Int64("ver", latestVer))
	} else {
	logutil.DDLLogger().Info("syncer check all versions, all nodes are not synced",
	zap.Int64("ddl job id", jobID),
	zap.Int64("ver", latestVer))
	}

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master