KVM: MMU: Fix is_dirty_pte()

is_dirty_pte() is used on guest ptes, not shadow ptes, so it needs to avoid
shadow_dirty_mask and use PT_DIRTY_MASK instead.

Misdetecting dirty pages could lead to unnecessarily setting the dirty bit
under EPT.

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

arch/x86/kvm/mmu.c

@@ -242,7 +242,7 @@ static int is_writeble_pte(unsigned long pte)
 
 static int is_dirty_pte(unsigned long pte)
 {
-	return pte & shadow_dirty_mask;
+	return pte & PT_DIRTY_MASK;
 }
 
 static int is_rmap_pte(u64 pte)