Kernel Heap Hardening: manage freed items with bitmaps rather than linked
lists when we're on-page and the page header is naturally big enough to
contain a bitmap.
This comes with no increase in memory consumption, and similar CPU cost
(maybe it's a little faster actually).
We want to favor bitmaps over linked lists, because linked lists install
kernel pointers inside the items, and this can be too easily exploitable
in use-after-free or double-free conditions, or in item buffer overflows
occurring within a pool page.
UnifiedSplitRaw
