sched/fair: skip smt expel when cpu down

jingdong inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I86JWB



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1. smt expel IPI is not needed for a dying cpu.

2. pick_next_task_fair cannot return NULL due to the smt expel
function during cpu down handler. Affects the migration of tasks.

Fixes: fd5207be ("sched: Implement the function of qos smt expeller")
Fixes: e4108f66 ("sched/fair: Introduce QOS_SMT_EXPELL priority reversion mechanism")
Signed-off-by: zhangwei123171 <zhangwei123171@jd.com>
Reviewed-by: zhaoxiaoqiang11 <zhaoxiaoqiang11@jd.com>