sched_ext: Do not enable LLC/NUMA optimizations when domains overlap

When the LLC and NUMA domains fully overlap, enabling both optimizations
in the built-in idle CPU selection policy is redundant, as it leads to
searching for an idle CPU within the same domain twice.

Likewise, if all online CPUs are within a single LLC domain, LLC
optimization is unnecessary.

Therefore, detect overlapping domains and enable topology optimizations
only when necessary.

Moreover, rely on the online CPUs for this detection logic, instead of
using the possible CPUs.

Fixes: 860a4521 ("sched_ext: Introduce NUMA awareness to the default idle selection policy")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>