cgroups: timer_slack: forward port timer_slack to 3.4. http://lwn.net/Articles/431847/

Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>
kernel: cgroup_timer_slack: Adaptations for 3.4 kernel

Signed-off-by: Cristoforo Cataldo <cristoforo.cataldo@gmail.com>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>
Let's enable it