From FreeBSD 11 both ip_len and ip_off fields of struct ip must be
provided in network byte order. See IP(4) on FreeBSD.

Change-Id: I091b551074767daf098cc1d6a256eddc4f55e304
Reviewed-on: https://go-review.googlesource.com/30736
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>