accounting: fix data race in Transferred()

startedTransfers is accessed by multiple threads, and it wasn't
protected by the mutex call in Transferred() func.

Fixes #4799

fs/accounting/stats.go

@@ -337,6 +337,8 @@ func (s *StatsInfo) String() string {
 // Transferred returns list of all completed transfers including checked and
 // failed ones.
 func (s *StatsInfo) Transferred() []TransferSnapshot {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
 	ts := make([]TransferSnapshot, 0, len(s.startedTransfers))
 
 	for _, tr := range s.startedTransfers {