virtio: make virtqueue_add_buf() returning 0 on success, not capacity.

 * Caller must ensure we don't call this with other virtqueue operations

 * at the same time (except where noted).

 *

 * Returns remaining capacity of queue or a negative error

 * (ie. ENOSPC).  Note that it only really makes sense to treat all

 * positive return values as "available": indirect buffers mean that

 * we can put an entire sg[] array inside a single queue entry.

 * Returns zero or a negative error (ie. ENOSPC, ENOMEM).

 */

int virtqueue_add_buf(struct virtqueue *_vq,

		      struct scatterlist sg[],

	pr_debug("Added buffer head %i to %p\n", head, vq);

	END_USE(vq);

	return vq->vq.num_free;

	return 0;

}

EXPORT_SYMBOL_GPL(virtqueue_add_buf);