libct/cg/sd/systemdVersion: don't return error

As the caller of this function just logs the error, it does not make
sense to pass it. Instead, log it (once) and return -1.

This is a preparation for the second user.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/common.go

@@ -34,7 +34,6 @@ var (
 
 	versionOnce sync.Once
 	version     int
-	versionErr  error
 
 	isRunningSystemdOnce sync.Once
 	isRunningSystemd     bool
@@ -372,19 +371,20 @@ func stopUnit(dbusConnection *systemdDbus.Conn, unitName string) error {
 	return nil
 }
 
-func systemdVersion(conn *systemdDbus.Conn) (int, error) {
+func systemdVersion(conn *systemdDbus.Conn) int {
 	versionOnce.Do(func() {
 		version = -1
 		verStr, err := conn.GetManagerProperty("Version")
-		if err != nil {
-			versionErr = err
-			return
+		if err == nil {
+			version, err = systemdVersionAtoi(verStr)
 		}
 
-		version, versionErr = systemdVersionAtoi(verStr)
+		if err != nil {
+			logrus.WithError(err).Error("unable to get systemd version")
+		}
 	})
 
-	return version, versionErr
+	return version
 }
 
 func systemdVersionAtoi(verStr string) (int, error) {
@@ -405,10 +405,8 @@ func systemdVersionAtoi(verStr string) (int, error) {
 func addCpuQuota(conn *systemdDbus.Conn, properties *[]systemdDbus.Property, quota int64, period uint64) {
 	if period != 0 {
 		// systemd only supports CPUQuotaPeriodUSec since v242
-		sdVer, err := systemdVersion(conn)
-		if err != nil {
-			logrus.Warnf("systemdVersion: %s", err)
-		} else if sdVer >= 242 {
+		sdVer := systemdVersion(conn)
+		if sdVer >= 242 {
 			*properties = append(*properties,
 				newProp("CPUQuotaPeriodUSec", period))
 		}