sched_ext: Add scx_bpf_task_set_slice() and scx_bpf_task_set_dsq_vtime()

sched_ext: Add scx_bpf_task_set_slice() and scx_bpf_task_set_dsq_vtime()

With the planned hierarchical scheduler support, sub-schedulers will need to
be verified for authority before being allowed to modify task->scx.slice and
task->scx.dsq_vtime. Add scx_bpf_task_set_slice() and
scx_bpf_task_set_dsq_vtime() which will perform the necessary permission
checks.

Root schedulers can still directly write to these fields, so this doesn't
affect existing schedulers.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>