vhost_net: use XDP helpers

Make use of the xdp_{init,prepare}_buff() helpers instead of
an open-coded version.

Also, the field xdp->rxq was never set, so pass NULL to xdp_init_buff()
to clear it.
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/net.c

@@ -744,11 +744,9 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
 	if (copied != len)
 		return -EFAULT;
 
-	xdp->data_hard_start = buf;
-	xdp->data = buf + pad;
-	xdp->data_end = xdp->data + len;
+	xdp_init_buff(xdp, buflen, NULL);
+	xdp_prepare_buff(xdp, buf, pad, len, true);
 	hdr->buflen = buflen;
-	xdp->frame_sz = buflen;
 
 	--net->refcnt_bias;
 	alloc_frag->offset += buflen;