accounting: fix --bwlimit when up or down is off - fixes #5019

Before this change the core bandwidth limit was limited to upload or
download value if the other value was off.

This fix only applies a core bandwidth limit when both values are set.

fs/accounting/token_bucket.go

@@ -68,7 +68,8 @@ func newTokenBucket(bandwidth fs.BwPair) (tbs buckets) {
 			bandwidthAccounting = bandwidth.Rx
 		}
 	}
-	if bandwidthAccounting > 0 {
+	// Limit core bandwidth to max of Rx and Tx if both are limited
+	if bandwidth.Tx > 0 && bandwidth.Rx > 0 {
 		tbs[TokenBucketSlotAccounting] = rate.NewLimiter(rate.Limit(bandwidthAccounting), maxBurstSize)
 	}
 	for _, tb := range tbs {