virtio_net: Use temporary storage for accounting rx stats

The purpose is to keep receive_buf arguments simple when more per-queue
counter items are added later.
Also XDP_TX related sq counters will be updated in the following changes
so create a container struct virtnet_rx_stats which will includes both
rq and sq statistics. For now it only covers rq stats.

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