sched_ext: Add the @sch parameter to ext_idle helpers

In preparation for multiple scheduler support, add the @sch parameter to
validate_node(), check_builtin_idle_enabled() and select_cpu_from_kfunc(),
and update their callers to read $scx_root, verify that it's not NULL and
pass it in. The passed in @sch parameter is not used yet.

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