cgroups: Introduce timer slack controller

Every task_struct has timer_slack_ns value. This value uses to round up
poll() and select() timeout values. This feature can be useful in
mobile environment where combined wakeups are desired.

Originally, prctl() was the only way to change timer slack value of
a process. So you was not able change timer slack value of another
process.

cgroup subsys "timer_slack" implements timer slack controller. It
provides a way to set minimal timer slack value for a group of tasks.
If a task belongs to a cgroup with minimal timer slack value higher than
task's value, cgroup's value will be applied.

Timer slack controller allows to implement setting timer slack value of
a process based on a policy. For example, you can create foreground and
background cgroups and move tasks between them based on system state.

Idea-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Cristoforo Cataldo <cristoforo.cataldo@gmail.com>
kernel: cgroup_timer_slack: Adaptations for 3.4 kernel

Signed-off-by: Cristoforo Cataldo <cristoforo.cataldo@gmail.com>
kernel: cgroup_timer_slack: Dynamic management of timer slack

This feature allows to use different minimal timer slack values and to
dynamically manage them when the system has been resumed (it's active)
or when has been suspended.

A new file is provided in addition to the two files provided by the
original timer slack controller:
- "timer_slack.min_slack_suspend_ns" is the minimal timer slack value
  to use when the system is suspended (screen off)

For further info, please refers to the timer slack documentation:
Documentation/cgroupd/timer_slack.txt

Signed-off-by: Cristoforo Cataldo <cristoforo.cataldo@gmail.com>