Now all non-deprecated RTA_* constants as of Linux v4.18 are present.

Change-Id: Ib04e233015f381ceb7c7e06b3d808f3c7948f5b4
Reviewed-on: https://go-review.googlesource.com/136456
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>