sched/fair: Use __this_cpu_read() in wake_wide()

The code is executed with preemption(and interrupts) disabled,
so it's safe to use __this_cpu_write().

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200421144123.33580-1-songmuchun@bytedance.com