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>