KVM: MMU: fix forgot flush tlbs on sync_page path

We should flush all tlbs after drop spte on sync_page path since

Quote from Avi:
| sync_page
| drop_spte
| kvm_mmu_notifier_invalidate_page
| kvm_unmap_rmapp
| spte doesn't exist -> no flush
| page is freed
| guest can write into freed page?

KVM-Stable-Tag.
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/paging_tmpl.h

@@ -786,6 +786,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 			else
 				nonpresent = shadow_notrap_nonpresent_pte;
 			drop_spte(vcpu->kvm, &sp->spt[i], nonpresent);
+			kvm_flush_remote_tlbs(vcpu->kvm);
 			continue;
 		}