virtio_net: fix virtnet_open and virtnet_probe competing for try_fill_recv

In function virtnet_open() and virtnet_probe(), func try_fill_recv() may
be executed at the same time. VQ in virtqueue_add() has not been protected
well and BUG_ON will be triggered when virito_net.ko being removed.

Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>