sched_ext: Enforce scheduler ownership when updating slice and dsq_vtime

sched_ext: Enforce scheduler ownership when updating slice and dsq_vtime

scx_bpf_task_set_slice() and scx_bpf_task_set_dsq_vtime() now verify that
the calling scheduler has authority over the task before allowing updates.
This prevents schedulers from modifying tasks that don't belong to them in
hierarchical scheduling configurations.

Direct writes to p->scx.slice and p->scx.dsq_vtime are deprecated and now
trigger warnings. They will be disallowed in a future release.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>