virtio_net: exploit napi_complete_done() return value

Since commit 364b6055

 ("net: busy-poll: return busypolling status to
drivers"), napi_complete_done() returns a boolean that can be used
by drivers to conditionally rearm interrupts.

This patch changes virtio_net to use this boolean to avoid a bit of
overhead for busy-poll users.

Jason reports about 1.1% improvement for 1 byte TCP_RR (burst 100).

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>