[kfence] Stolen freelists are CPU-local

[ramosian-glider]

SLUB uses per-CPU freelists on allocation fast path, so we need to ensure we steal evenly from each CPU. kfence_steal_freelist() always steals the freelist from the current CPU, and kfence_fix_freelist()fixes it for the current CPU, even if the freelist was stolen from another CPU. We may want to do one of the following:

• steal every cache->cpu_slab->freelist for the cache in kfence_steal_freelist(). In that case every allocation from that cache on every other CPU will be waiting for kfence_alloc_and_fix_freelist() to finish and restore the freelists.
• pick a random CPU in kfence_steal_freelist() and steal only the corresponding freelist.

[melver]

I think the fairest result will be obtained by picking a random CPU. Fair as in on every system, regardless of number of CPUs, adjusting the sample rate has a predictable result on system behaviour without having to think about number of CPUs and factoring that into what the sample rate should be.

[ramosian-glider]

Should be fine now.