sched/fair: Introduce multiple qos level

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8S4JH



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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.

Signed-off-by: Zhao Wenhui <zhaowenhui8@huawei.com>