sched: Expose css_tg()

A new BPF extensible sched_class will use css_tg() in the init and exit
paths to visit all task_groups by walking cgroups.

v4: __setscheduler_prio() is already exposed. Dropped from this patch.

v3: Dropped SCHED_CHANGE_BLOCK() as upstream is adding more generic cleanup
    mechanism.

v2: Expose SCHED_CHANGE_BLOCK() too and update the description.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>