vdpa: Fix pointer math bug in vdpasim_get_config()

If "offset" is non-zero then we end up copying from beyond the end of
the config because of pointer math.  We can fix this by casting the
struct to a u8 pointer.

Fixes: 2c53d0f64c ("vdpasim: vDPA device simulator")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200406144552.GF68494@mwanda
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -558,7 +558,7 @@ static void vdpasim_get_config(struct vdpa_device *vdpa, unsigned int offset,
 	struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
 
 	if (offset + len < sizeof(struct virtio_net_config))
-		memcpy(buf, &vdpasim->config + offset, len);
+		memcpy(buf, (u8 *)&vdpasim->config + offset, len);
 }
 
 static void vdpasim_set_config(struct vdpa_device *vdpa, unsigned int offset,