caif_virtio: fix race between virtio_device_ready() and ndo_open()

We currently depend on probe() calling virtio_device_ready() -
which happens after netdev
registration. Since ndo_open() can be called immediately
after register_netdev, this means there exists a race between
ndo_open() and virtio_device_ready(): the driver may start to use the
device (e.g. TX) before DRIVER_OK which violates the spec.

Fix this by switching to use register_netdevice() and protect the
virtio_device_ready() with rtnl_lock() to make sure ndo_open() can
only be called after virtio_device_ready().

Fixes: 0d2e1a29 ("caif_virtio: Introduce caif over virtio")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220620051115.3142-3-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>