Revert "KVM: Avoid useless memory write when possible"

This reverts commit 8a1449563b3e5ede56b28cc977c8da22a17cdf51.  While it
does save useless updates, it (probably) defeats the fork detector, causing
a massive performance loss.

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index 7c9bd72..6e11871 100644
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -1085,10 +1085,8 @@
 		return 0;
 	mark_page_dirty(vcpu->kvm, gpa >> PAGE_SHIFT);
 	virt = kmap_atomic(page, KM_USER0);
-	if (memcmp(virt + offset_in_page(gpa), val, bytes)) {
-		kvm_mmu_pte_write(vcpu, gpa, virt + offset, val, bytes);
-		memcpy(virt + offset_in_page(gpa), val, bytes);
-	}
+	kvm_mmu_pte_write(vcpu, gpa, virt + offset, val, bytes);
+	memcpy(virt + offset_in_page(gpa), val, bytes);
 	kunmap_atomic(virt, KM_USER0);
 	return 1;
 }