[NET]: Expose netdevice dev_id through sysfs

Expose dev_id to userspace, because it helps to disambiguate between
interfaces where the MAC address is unique.

This should allow us to simplify the handling of persistent naming for
S390 network devices in udev -- because it can depend on a simple
attribute of the device like the other match criteria, rather than
having a special case for SUBSYSTEMS=="ccwgroup".

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>