Starting with 1.12, we must use syscall versions
of sysctl instead of the raw syscall.

An identical CL went into the source copy at golang.org/x/net/route.
This is just a cherry pick of that CL.
(CL: https://go-review.googlesource.com/c/net/+/148597)

Change-Id: I6286ab3e49f82512491afb5bcf349e89ab5645ab
Reviewed-on: https://go-review.googlesource.com/c/149637Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>