mm: Dynamic management of dirty page writebacks

This feature allows to have two different intervals for dirty page
writebacks and to dynamically manage them when the system has been
resumed (it's active) or when has been suspended.

Three new procfs parameters are exposed inside /proc/sys/vm path:
- "dynamic_dirty_writeback" is the activation status of this feature,
  set 1 to enable it, set 0 to disable it and use the standard behaviour
- "dirty_writeback_active_centisecs" is the interval for the dirty page
  writebacks when the system is active (screen on)
- "dirty_writeback_suspend_centisecs" is the interval for the dirty page
  writebacks when the system is suspended (screen off)

This feature is in part inspired on Francisco Franco's patch:
https://github.com/franciscofranco/mako/commit/34d79548cf16b69c7d0154836018e7f258a22947

Signed-off-by: Cristoforo Cataldo <cristoforo.cataldo@gmail.com>
Signed-off-by: Dan Trifan <jimsth@gmail.com>
defconfig: enable DYNAMIC_PAGE_WRITEBACK

Signed-off-by: Dan Trifan <jimsth@gmail.com>
mm: finetune the filesystem dynamic read-ahead

Signed-off-by: Dan Trifan <jimsth@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>
Signed-off-by: Dan Trifan <jimsth@gmail.com>
defconfig: enable dynamic CGROUP_TIMER_SLACK

Signed-off-by: Dan Trifan <jimsth@gmail.com>