veth: Account for packet drops in ndo_xdp_xmit

Use existing atomic drop counter. Since drop path is really an
exceptional case here, I'm thinking atomic ops would not hurt the
performance.
XDP packets and bytes are not counted in ndo_xdp_xmit, but will be
accounted on rx side by the following commit.

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>