[S390] sparse: fix sparse symbol shadow warning

to_kvmdev and dev_to_virtio both use container_of. Avoid to nest the
two macros to quiet the following sparse warning:

drivers/s390/kvm/kvm_virtio.c:337:20: warning: symbol '__mptr' shadows an earlier one
drivers/s390/kvm/kvm_virtio.c:337:20: originally declared here

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/kvm/kvm_virtio.c

@@ -334,10 +334,10 @@ static void scan_devices(void)
  */
 static int match_desc(struct device *dev, void *data)
 {
-	if ((ulong)to_kvmdev(dev_to_virtio(dev))->desc == (ulong)data)
-		return 1;
+	struct virtio_device *vdev = dev_to_virtio(dev);
+	struct kvm_device *kdev = to_kvmdev(vdev);
 
-	return 0;
+	return kdev->desc == data;
 }
 
 /*