KVM x86 MMU changes for 6.7:

 - Clean up code that deals with honoring guest MTRRs when the VM has
   non-coherent DMA and host MTRRs are ignored, i.e. EPT is enabled.

 - Zap EPT entries when non-coherent DMA assignment stops/start to prevent
   using stale entries with the wrong memtype.

 - Don't ignore guest PAT for CR0.CD=1 && KVM_X86_QUIRK_CD_NW_CLEARED=y, as
   there's zero reason to ignore guest PAT if the effective MTRR memtype is WB.
   This will also allow for future optimizations of handling guest MTRR updates
   for VMs with non-coherent DMA and the quirk enabled.

 - Harden the fast page fault path to guard against encountering an invalid
   root when walking SPTEs.