sched_ext: idle: small CPU iteration refactoring

sched_ext: idle: small CPU iteration refactoring

Replace the loop to check if all SMT CPUs are idle with
cpumask_subset(). This simplifies the code and slightly improves
efficiency, while preserving the original behavior.

Note that idle_masks.smt handling remains racy, which is acceptable as
it serves as an optimization and is self-correcting.

Suggested-and-reviewed-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>