sched: programmable: Add convenient helper functions to convert sched entity

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5KUFB


CVE: NA

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Add three helper functions:
1) bpf_sched_entity_is_task is to check whether the sched entity
is a task struct.
2) bpf_sched_entity_to_task is to change the sched entity to a
task struct.
3) bpf_sched_entity_to_tg is to change the sched entity to a task
group.

Signed-off-by: Chen Hui <judy.chenhui@huawei.com>
Signed-off-by: Ren Zhijie <renzhijie2@huawei.com>
Signed-off-by: Hui Tang <tanghui20@huawei.com>