libct/cg/sd/dbus: fix NewDbusConnManager

Noticed that the check of trying to use both rootful and rootless
in NewDbusConnManager never worked, as we never set dbusInited to true.

Do that. While at it, protect this with the mutex (against the
case of two goroutines simultaneously calling NewDbusConnManager).
This is a rare call, so taking read-only then read-write mutex does not
make sense.

Fixes: c7f847e

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

libcontainer/cgroups/systemd/dbus.go

@@ -21,9 +21,12 @@ type dbusConnManager struct{}
 
 // newDbusConnManager initializes systemd dbus connection manager.
 func newDbusConnManager(rootless bool) *dbusConnManager {
+	dbusMu.Lock()
+	defer dbusMu.Unlock()
 	if dbusInited && rootless != dbusRootless {
 		panic("can't have both root and rootless dbus")
 	}
+	dbusInited = true
 	dbusRootless = rootless
 	return &dbusConnManager{}
 }