- Fix an undefined behavior warning where a crafty userspace can read kvm.ko's
nx_huge_pages before it's fully initialized.
- Don't zero-allocate page tables that are used for splitting hugepages in the
TDP MMU, as KVM is guaranteed to set all SPTEs in the page table and thus
write all bytes.
- Bail early when trying to unsync 4KiB mappings if the target gfn can be
mapped with a 2MiB hugepage, to avoid the gfn hash lookup.