We are dropping A/D bits (and W bits) in the TDP MMU.  Even if mmu_lock
is held for write, as volatile SPTEs can be written by other tasks/vCPUs
outside of mmu_lock.

Attempting to prove that bug exposed another notable goof, which has been
lurking for a decade, give or take: KVM treats _all_ MMU-writable SPTEs
as volatile, even though KVM never clears WRITABLE outside of MMU lock.
As a result, the legacy MMU (and the TDP MMU if not fixed) uses XCHG to
update writable SPTEs.

The fix does not seem to have an easily-measurable affect on performance;
page faults are so slow that wasting even a few hundred cycles is dwarfed
by the base cost.