!795 sched/fair: Introduce multiple qos level

Merge Pull Request from: @zhaowenhui8 
 
Expand qos_level from {-1,0} to [-2, 2], to distinguish the tasks expected
to be with extremely high or low priority level. Using qos_level_weight
to reweight the shares when calculating group's weight. Meanwhile,
set offline task's schedule policy to SCHED_IDLE so that it can be
preempted at check_preempt_wakeup.

kernel option:
CONFIG_QOS_SCHED_MULTILEVEL 
 
Link:https://gitee.com/openeuler/kernel/pulls/795

 

Reviewed-by: Zucheng Zheng <zhengzucheng@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>