udev: net_id - support predictable ifnames on vio buses (#5675)

udev: net_id - support predictable ifnames on vio buses (#5675)

For IBM PowerVM Virtual I/O network devices, we can build predictable names
based on the slot number passed as part of the OF "reg" property.  Valid slot
numbers range between 2-32767, so we only need the bottom half of the unit
address passed.

For example:

  /proc/device-tree/vdevice/l-lan@30000002
  /proc/device-tree/vdevice/vnic@30000005

would initially map to something like:

  /sys/devices/vio/30000002/net/eth0
  /sys/devices/vio/30000005/net/eth1

and would then translate to env2 and env5

This patch ignores the bus number, as there should only ever be one bus, and
then remove leading zeros.