!877 sched: fix performance degradation on lmbench

Merge Pull Request from: @barry19901226 
 
There are worse performance with the 'Fixes'
when running "./lat_ctx -P $SYNC_MAX -s 64 16".

The 'Fixes' which allocates memory for p->prefer_cpus
even if "prefer_cpus" not be set.

Before the 'Fixes', only test "p->prefer_cpus",
after, add test "!cpumask_empty(p->prefer_cpus)"
which causing performance degradation.

select_task_rq_fair
  ->set_task_select_cpus
    ->prefer_cpus_valid  ----  test cpumask_empty(p->prefer_cpus) 
 
Link:https://gitee.com/openeuler/kernel/pulls/877

 

Reviewed-by: Zucheng Zheng <zhengzucheng@huawei.com>
Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>