virtio-net: fix recursived rtnl_lock() during probe()

In the Linux kernel, the following vulnerability has been resolved:

virtio-net: fix recursived rtnl_lock() during probe()

The deadlock appears in a stack trace like:

virtnet_probe()

rtnl_lock()

virtio_config_changed_work()

netdev_notify_peers()

rtnl_lock()

It happens if the VMM sends a VIRTIO_NET_S_ANNOUNCE request while the

virtio-net driver is still probing.

The config_work in probe() will get scheduled until virtnet_open() enables

the config change notification via virtio_config_driver_enable().