Built with 3.18 linux kernel headers on Gentoo.

Change-Id: Ic21a315eb7569c6682f5cb2c991f0966245197f9
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/10044Reviewed-by: Rob Pike <r@golang.org>