KVM: MMU: Fix race when instantiating a shadow pte

For improved concurrency, the guest walk is performed concurrently with other
vcpus.  This means that we need to revalidate the guest ptes once we have
write-protected the guest page tables, at which point they can no longer be
modified.

The current code attempts to avoid this check if the shadow page table is not
new, on the assumption that if it has existed before, the guest could not have
modified the pte without the shadow lock.  However the assumption is incorrect,
as the racing vcpu could have modified the pte, then instantiated the shadow
page, before our vcpu regains control:

  vcpu0        vcpu1

  fault
  walk pte

               modify pte
               fault in same pagetable
               instantiate shadow page

  lookup shadow page
  conclude it is old
  instantiate spte based on stale guest pte

We could do something clever with generation counters, but a test run by
Marcelo suggests this is unnecessary and we can just do the revalidation
unconditionally.  The pte will be in the processor cache and the check can
be quite fast.

Signed-off-by: Avi Kivity <avi@qumranet.com>