KVM: MMU: fix mmu notifier invalidate handler for huge spte

The index wasn't calculated correctly (off by one) for huge spte so KVM guest
was unstable with transparent hugepages.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>