veth: introduce more xdp counters

Introduce xdp_xmit counter in order to distinguish between XDP_TX and
ndo_xdp_xmit stats. Introduce the following ethtool counters:
- rx_xdp_tx
- rx_xdp_tx_errors
- tx_xdp_xmit
- tx_xdp_xmit_errors
- rx_xdp_redirect

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>