unix/linux: extend rtnetlink constants

Change-Id: Icc356897519c0aa229a948918a253b9e4b367aff
Reviewed-on: https://go-review.googlesource.com/c/sys/+/697795
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>