mm: Added dynamic writeback (again!)

So i did some research and before this patch the wirteback
deamon flushes every 600 ms dirty pages. With this patch
it flashes only every second while screen is on and every 300
ms while screen is off. Also the expire_interval changes dynamically.

Change-Id: I579e91db4c73898f30a59b7648d83767d1031296

Conflicts:
	mm/page-writeback.c