sched_ext: Fix ops.cgroup_move() invocation kf_mask and rq tracking

sched_move_task() invokes ops.cgroup_move() inside task_rq_lock(tsk), so
@p's rq lock is held. The SCX_CALL_OP_TASK invocation mislabels this:

- kf_mask = SCX_KF_UNLOCKED (== 0), claiming no lock is held.
- rq = NULL, so update_locked_rq() doesn't run and scx_locked_rq()
returns NULL.

Switch to SCX_KF_REST and pass task_rq(p), matching ops.set_cpumask()
from set_cpus_allowed_scx().

Three effects:

- scx_bpf_task_cgroup() becomes callable (was rejected by
scx_kf_allowed(__SCX_KF_RQ_LOCKED)). Safe; rq lock is held.

- scx_bpf_dsq_move() is now rejected (was allowed via the unlocked
branch). Calling it while holding an unrelated task's rq lock is
risky; rejection is correct.

- scx_bpf_select_cpu_*() previously took the unlocked branch in
select_cpu_from_kfunc() and called task_rq_lock(p, &rf), which
would deadlock against the already-held pi_lock. Now it takes the
locked-rq branch and is rejected with -EPERM via the existing
kf_allowed(SCX_KF_SELECT_CPU | SCX_KF_ENQUEUE) check. Latent
deadlock fix.

No in-tree scheduler is known to call any of these from ops.cgroup_move().

v2: Add Fixes: tag (Andrea Righi).

Fixes: 18853ba782be ("sched_ext: Track currently locked rq")
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>