The cgroupstats structure is available via Linux's generic netlink
taskstats interface.

For more information, please see:
https://www.kernel.org/doc/Documentation/accounting/cgroupstats.txt.

Fixes golang/go#23388.

Change-Id: I8a05a3875209e99a427d54049eacc98f415e44cf
Reviewed-on: https://go-review.googlesource.com/86975
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>