udev/net_id: use STRLEN() to make code clearer

udev/net_id: use STRLEN() to make code clearer

The code was correct, but looked suspicious: we were comparing
strlen(x) with sizeof(y), with looks like an off-by-one. But we actually
want x to be one longer than y, so that's fine. Let's use STRLEN() to
make this more obvious.

While at it, drop unnecessary "_" prefix.