improve the robustness of slub page freelists

[thestinger]

Ideally, out-of-line metadata like bitmaps would be used. However, progress can be made without any drastic changes particularly when canaries are enabled. It would be nice to have fast range-checking code, similar to the existing debug code but likely without the expensive % checking alignment or at least it should be micro-optimized. It also needs to check in more places than the debug code to have the full intended security value.