virtio-net: introduce virtnet_receive()

Move common receive logic to a new helper virtnet_receive(). It will
also be used by rx busy polling method.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Vlad Yasevich <vyasevic@redhat.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>