scsi: virtio_scsi: Use struct_size() helper

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct virtio_scsi {
	...
        struct virtio_scsi_vq req_vqs[];
};

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following form:

sizeof(*vscsi) + sizeof(vscsi->req_vqs[0]) * num_queues

with:

struct_size(vscsi, req_vqs, num_queues)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

drivers/scsi/virtio_scsi.c

@@ -795,7 +795,7 @@ static int virtscsi_probe(struct virtio_device *vdev)
 	num_targets = virtscsi_config_get(vdev, max_target) + 1;
 
 	shost = scsi_host_alloc(&virtscsi_host_template,
-		sizeof(*vscsi) + sizeof(vscsi->req_vqs[0]) * num_queues);
+				struct_size(vscsi, req_vqs, num_queues));
 	if (!shost)
 		return -ENOMEM;