net/dns: slightly optimize dbusPing for non-dbus case [Linux]

Avoid some work when D-Bus isn't running.

Change-Id: I6f89bb75fdb24c13f61be9b400610772756db1ef
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager_linux.go

@@ -321,15 +321,15 @@ func resolvedIsActuallyResolver(bs []byte) error {
 }
 
 func dbusPing(name, objectPath string) error {
-	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
-	defer cancel()
-
 	conn, err := dbus.SystemBus()
 	if err != nil {
 		// DBus probably not running.
 		return err
 	}
 
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+
 	obj := conn.Object(name, dbus.ObjectPath(objectPath))
 	call := obj.CallWithContext(ctx, "org.freedesktop.DBus.Peer.Ping", 0)
 	return call.Err