scx_layered: Restrict preemption to layer cpumask

[hodgesds]

When preempting restrict preemption to the current layer cpumask. This may reduce the amount of preemption, but cause better cache locality of preempted tasks.

[hodgesds]

drive by fix, this check is wrong and not needed

[likewhatevs]

lgtm, a comment explaining the mask manipulation would be nice though

[hodgesds]

Tested with stress-ng and I can see most no X-NUMA preemptions with a config to keep the stress-ng tasks on a single node:

###### Tue, 24 Sep 2024 07:16:56 -0700 ######
tot=  11971 local=44.30 open_idle= 0.00 affn_viol= 0.78 proc=6ms
busy= 15.6 util= 1114.5 load=   3867.2 fallback_cpu=  2
excl_coll=10.02 excl_preempt=0.13 excl_idle=0.11 excl_wakeup=0.00
  hodgesd  : util/frac=    4.4/  0.4 load/frac=      5.5/  0.1 tasks=   829
             tot=   4366 local=94.46 wake/exp/reenq= 5.52/ 0.02/ 0.00
             xlayer_wake= 4.63 xlayer_rewake= 0.53
             keep/max/busy= 0.89/ 0.00/ 0.09 kick= 0.00 yield/ign= 0.05/    0 
             open_idle= 0.00 mig= 7.47 xnuma_mig= 0.53 xllc_mig= 0.53 affn_viol= 0.00
             preempt/first/xllc/xnuma/idle/fail= 0.00/ 0.00/ 0.00/ 0.00/ 0.00/ 0.00 min_exec= 0.00/   0.00ms
             cpus=  2 [  2,  2] 00001000 00100000 00000000
             excl_coll= 0.00 excl_preempt= 0.00
  normal   : util/frac=  141.4/ 12.7 load/frac=   2861.7/ 74.0 tasks=  4549
             tot=   7589 local=15.54 wake/exp/reenq=83.70/ 0.76/ 0.00
             xlayer_wake= 6.26 xlayer_rewake= 1.25
             keep/max/busy=16.06/ 0.00/ 0.00 kick= 0.58 yield/ign= 0.42/  356 
             open_idle= 0.00 mig=77.23 xnuma_mig=12.37 xllc_mig=12.37 affn_viol= 1.23
             preempt/first/xllc/xnuma/idle/fail= 0.41/ 0.25/ 0.00/ 0.00/69.43/28.09 min_exec= 0.00/   0.00ms
             cpus=  4 [  2,  4] 00000003 00000300 00000000
             excl_coll=15.81 excl_preempt= 0.20
  random   : util/frac=    0.0/  0.0 load/frac=      0.0/  0.0 tasks=     0
             tot=      0 local= 0.00 wake/exp/reenq= 0.00/ 0.00/ 0.00
             xlayer_wake= 0.00 xlayer_rewake= 0.00
             keep/max/busy= 0.00/ 0.00/ 0.00 kick= 0.00 yield/ign= 0.00/    0 
             open_idle= 0.00 mig= 0.00 xnuma_mig= 0.00 xllc_mig= 0.00 affn_viol= 0.00
             preempt/first/xllc/xnuma/idle/fail= 0.00/ 0.00/ 0.00/ 0.00/ 0.00/ 0.00 min_exec= 0.00/   0.00ms
             cpus=  0 [  0,  0] 00000000 00000000 00000000
  stress-ng: util/frac=  968.7/ 86.9 load/frac=   1000.0/ 25.9 tasks=    11
             tot=     16 local= 0.00 wake/exp/reenq= 0.00/100.0/ 0.00
             xlayer_wake= 0.00 xlayer_rewake= 0.00
             keep/max/busy=66187.5/125.0/ 0.00 kick=100.0 yield/ign= 0.00/    0 
             open_idle= 0.00 mig=87.50 xnuma_mig=31.25 xllc_mig=31.25 affn_viol= 0.00
             preempt/first/xllc/xnuma/idle/fail= 0.00/ 0.00/ 0.00/ 0.00/ 0.00/ 0.00 min_exec= 0.00/   0.00ms
             cpus= 14 [ 14, 14] 07f00000 f0000000 00000007
             excl_coll= 0.00 excl_preempt= 0.00