KVM: MMU: fix sync of ptes addressed at owner pagetable

[Marcelo Tosatti <mtosatti@redhat.com>]

During page sync, if a pagetable contains a self referencing pte (that
points to the pagetable), the corresponding spte may be marked as
writable even though all mappings are supposed to be write protected.

Fix by clearing page unsync before syncing individual sptes.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 0813a08..8904e8a 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1046,13 +1046,13 @@ static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 	}
 
 	rmap_write_protect(vcpu->kvm, sp->gfn);
+	kvm_unlink_unsync_page(vcpu->kvm, sp);
 	if (vcpu->arch.mmu.sync_page(vcpu, sp)) {
 		kvm_mmu_zap_page(vcpu->kvm, sp);
 		return 1;
 	}
 
 	kvm_mmu_flush_tlb(vcpu);
-	kvm_unlink_unsync_page(vcpu->kvm, sp);
 	return 0;
 }