sched/rt: Fix possible warn when push_rt_task

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7FJB5



-------------------------------

A warn may be triggered during reboot, as follows:

reboot
  ->kernel_restart
    ->machine_restart
      ->smp_send_stop --- ipi handler set_cpu_online(cpu, false)

balance_callback
-> __balance_callback
  ->push_rt_task
    -> find_lock_lowest_rq  --- offline cpu in vec->mask not be cleared
      -> find_lowest_rq
        -> cpupri_find
          -> cpupri_find_fitness
            -> __cpupri_find [cpumask_and(..., vec->mask)]
    -> set_task_cpu(next_task, lowest_rq->cpu) --- WARN_ON(!oneline(cpu)

So add !cpu_online(lowest_rq->cpu) check before set_task_cpu().
The fix does not completely fix the problem, since cpu_online_mask may
be cleared after check.

Fixes: 4ff9083b ("sched/core: WARN() when migrating to an offline CPU")
Signed-off-by: Hui Tang <tanghui20@huawei.com>