virtio-net: correctly update XDP_TX counters

Commit 5b8f3c8d ("virtio_net: Add XDP related stats") tries to
count TX XDP stats in virtnet_receive(). This will cause several
issues:

- virtnet_xdp_sq() was called without checking whether or not XDP is
  set. This may cause out of bound access when there's no enough txq
  for XDP.
- Stats were updated even if there's no XDP/XDP_TX.

Fixing this by reusing virtnet_xdp_xmit() for XDP_TX which can counts
TX XDP counter itself and remove the unnecessary tx stats embedded in
rx stats.

Reported-by:  <syzbot+604f8271211546f5b3c7@syzkaller.appspotmail.com>
Fixes: 5b8f3c8d ("virtio_net: Add XDP related stats")
Cc: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>