Add all IFLA_* constants defined in linux/if_link.h as of Linux kernel
version 4.15.

Change-Id: Ib27205cf560f40303ca9500abe73abf833fc13a8
Reviewed-on: https://go-review.googlesource.com/96595
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>