net: handle more interface types without a hardware address on Plan 9

We were handling loopback devices when attempting to read hardware
address, but packet interfaces were not being handled. As a general fix,
don't attempt to read hardware address of any device that's not inside
/net.

Fixes #39908

Change-Id: Ifa05e270357e111c60906110db2cc23dc7c1c49c
Reviewed-on: https://go-review.googlesource.com/c/go/+/240259


Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>