Fix split lock issue on MappedSessionStatistics

[tonytamwk]

• add 4-bytes padding

Run the following program on a CPU with split lock detection feature lscpu | grep split_loc and see the dmesg

    public static void main(String[] args) {
        MappedSessionStatistics stats = new MappedSessionStatistics();
        stats.messageSent(); // dmesg: [  180.886150] x86/split lock detection: #AC: java/10167 took a split_lock trap at address: 0x3ff2624d
        System.out.println(stats.messagesSent());
    }

https://docs.kernel.org/arch/x86/buslock.html