MulticastSocket has two ways to report whether loopback of multicast datagrams is enabled or disabled:
1. Legacy getLoopbackMode(), should return true when loopback is disabled.
2. getOption(IP_MULTICAST_LOOP), should return true when loopback is enabled.
We seem to have a bug with #2 in that it returns true when the loopback is disabled. These methods should always return the inverse of one another.
The setOption/getOption methods are @since 9 so the compatibility impact of fixing this is probably low.