sched_ext: Introduce scx_prog_sched()

In preparation for multiple scheduler support, introduce scx_prog_sched()
accessor which returns the scx_sched instance associated with a BPF program.
The association is determined via the special KF_IMPLICIT_ARGS kfunc
parameter, which provides access to bpf_prog_aux. This aux can be used to
retrieve the struct_ops (sched_ext_ops) that the program is associated with,
and from there, the corresponding scx_sched instance.

For compatibility, when ops.sub_attach is not implemented (older schedulers
without sub-scheduler support), unassociated programs fall back to scx_root.
A warning is logged once per scheduler for such programs.

As scx_root is still the only scheduler, this shouldn't introduce
user-visible behavior changes.

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

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