virtio-mem: Don't rely on implicit compiler padding for requests

The compiler will add padding after the last member, make that explicit.
The size of a request is always 24 bytes. The size of a response always
10 bytes. Add compile-time checks.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: teawater <teawaterz@linux.alibaba.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200515101402.16597-1-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mem.c

@@ -1770,6 +1770,9 @@ static int virtio_mem_probe(struct virtio_device *vdev)
 	struct virtio_mem *vm;
 	int rc = -EINVAL;
 
+	BUILD_BUG_ON(sizeof(struct virtio_mem_req) != 24);
+	BUILD_BUG_ON(sizeof(struct virtio_mem_resp) != 10);
+
 	vdev->priv = vm = kzalloc(sizeof(*vm), GFP_KERNEL);
 	if (!vm)
 		return -ENOMEM;

include/uapi/linux/virtio_mem.h

@@ -103,16 +103,19 @@
 struct virtio_mem_req_plug {
 	__virtio64 addr;
 	__virtio16 nb_blocks;
+	__virtio16 padding[3];
 };
 
 struct virtio_mem_req_unplug {
 	__virtio64 addr;
 	__virtio16 nb_blocks;
+	__virtio16 padding[3];
 };
 
 struct virtio_mem_req_state {
 	__virtio64 addr;
 	__virtio16 nb_blocks;
+	__virtio16 padding[3];
 };
 
 struct virtio_mem_req {