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>
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