virtio-net: correctly set xdp_xmit for mergeable buffer

We should set xdp_xmit only when xdp_do_redirect() succeed.

Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>