tso: fix an invalid err != nil check (tikv#8523)

close tikv#8524

Use the continue label to retry the whole merge loop correctly.

Signed-off-by: JmPotato <ghzpotato@gmail.com>

pkg/tso/keyspace_group_manager.go

@@ -1304,6 +1304,7 @@ func (kgm *KeyspaceGroupManager) mergingChecker(ctx context.Context, mergeTarget
 		mergeMap[id] = struct{}{}
 	}
 
+mergeLoop:
 	for {
 		select {
 		case <-ctx.Done():
@@ -1375,15 +1376,13 @@ func (kgm *KeyspaceGroupManager) mergingChecker(ctx context.Context, mergeTarget
 					zap.Uint32("merge-id", id),
 					zap.Time("ts", ts),
 					zap.Error(err))
-				break
+				// Retry from the beginning of the loop.
+				continue mergeLoop
 			}
 			if ts.After(mergedTS) {
 				mergedTS = ts
 			}
 		}
-		if err != nil {
-			continue
-		}
 		// Update the newly merged TSO if the merged TSO is not zero.
 		if mergedTS != typeutil.ZeroTime {
 			log.Info("start to set the newly merged TSO",