scx_layered: Fix idle core selection

[hodgesds]

Fix idle core selection to correctly use pick_idle_cpu_from. Previously it was passing in the mask of the topology (LLC/node) rather than the idle mask of the topology. This will be refactored in the future when struct layer_cpumask_wrapper gets support for additional bpf_cpumasks for topologies.

[hodgesds]

Tested with @JakeHillion with following config:

[{
  "name":"random",
  "comment":"random user",
  "matches":[
     [{"UIDEquals":1234}]
  ],
  "kind": {
    "Confined":{
      "nodes":[1],
      "util_range": [0.25, 0.7],
      "growth_algo": "Random",
      "llcs": [1, 0],
      "preempt": false,
      "slice_us": 500,
      "exclusive": false
     }
  }
},
{
  "name":"hodgesd",
  "comment":"hodgesd user",
  "matches":[
     [{"UIDEquals":224791}]
  ],
  "kind": {
    "Confined":{
      "slice_us": 800,
      "util_range": [0.25, 0.6],
      "growth_algo": "Topo",
      "nodes": [1, 0],
      "preempt": true,
      "preempt_first": true,
      "exclusive": true
     }
  }
},{
  "name":"stress-ng",
  "comment":"stress-ng slice",
  "matches":[
     [
	     {"CommPrefix":"stress-ng"}
     ],[
	     {"PcommPrefix":"stress-ng"}
     ]],
  "kind": {
    "Confined":{
      "util_range": [0.05, 0.80],
      "slice_us": 800,
      "preempt": false,
      "preempt_first": false,
      "growth_algo": "RoundRobin",
      "exclusive": true
     }
  }
},
{
  "name":"normal",
  "comment":"the rest",
  "matches":[[]],
  "kind":{
    "Confined": {
      "util_range": [0.25, 0.6],
      "preempt": true,
      "preempt_first": true,
      "slice_us": 800,
      "exclusive": true,
      "growth_algo": "Sticky"
    }
  }
}]

stress-ng gets spread across NUMA nodes properly:

[JakeHillion]

Tested on an 11 CCX machine with RoundRobin. Seems to work great with this change, thanks for the fix!