disable preemption when update ktap_stats

Kernel reported below warning in preempt kernel.

    BUG: using smp_processor_id() in preemptible [00000000] code: ktap/1865
    caller is kp_malloc+0xbd/0xd6 [ktapvm]
    CPU: 11 PID: 1865 Comm: ktap Tainted: G           O 3.12.0-rc6+ #5

The reason is we didn't disable preemption when update
percpu variable ktap_stats, this patch fix it.

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>