virtio_ring: allow to store zero as the ctx

Allow zero to be store as a ctx, with this we could store e.g zero
value which could be meaningful for the case of storing headroom
through ctx.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/virtio/virtio_ring.c

@@ -391,7 +391,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	vq->desc_state[head].data = data;
 	if (indirect)
 		vq->desc_state[head].indir_desc = desc;
-	if (ctx)
+	else
 		vq->desc_state[head].indir_desc = ctx;
 
 	/* Put entry in available array (but don't update avail->idx until they