sched_ext: Drop redundant rq-locked check from scx_bpf_task_cgroup()

sched_ext: Drop redundant rq-locked check from scx_bpf_task_cgroup()

scx_kf_allowed_on_arg_tasks() runs both an scx_kf_allowed(__SCX_KF_RQ_LOCKED)
mask check and a kf_tasks[] check. After the preceding call-site fixes,
every SCX_CALL_OP_TASK*() invocation has kf_mask & __SCX_KF_RQ_LOCKED
non-zero, so the mask check is redundant whenever the kf_tasks[] check
passes. Drop it and simplify the helper to take only @sch and @p.

Fold the locking guarantee into the SCX_CALL_OP_TASK() comment block, which
scx_bpf_task_cgroup() now points to.

No functional change.

Extracted from a larger verifier-time kfunc context filter patch
originally written by Juntong Deng.

Original-patch-by: Juntong Deng <juntong.deng@outlook.com>
Cc: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>