[NET]: Add probe_old_netdevs() hook.

[NET]: Add probe_old_netdevs() hook.

This set of patches is a mixture of Al's work to device initialization,
and some of my own to complete it for all the ether, tr, sbni, and loopback
devices.

The first patch adds the hook for converting old driver initialization
code over to dynamic allocation.

This part extracted from Al viro's set of net driver changes for ethertap.