sched: fix performance degradation on lmbench

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7A718



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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)

Fixes: ebeb84ad ("cpuset: Introduce new interface for scheduler ...")
Signed-off-by: Hui Tang <tanghui20@huawei.com>