mm: vmpressure: make vmpressure window variable

Right now the vmpressure window is of constant size 2MB, which
works well with the following exceptions.
1) False vmpressure triggers are seen when the RAM size is greater
than 3GB. This results in lowmemorykiller, which uses vmpressure
events, killing tasks unnecessarily.
2) Vmpressure events are received late under memory pressure. This
behaviour is seen prominently in <=2GB RAM targets. This results in
lowmemorykiller kicking in late to kill tasks resulting in avoidable
page cache reclaim.

The problem analysis shows that the issue is with the constant size
of the vmpressure window which does not adapt to the varying memory
conditions. This patch recalculates the vmpressure window size at
the end of each window. The chosen window size is proportional to
the total of free and cached memory at that point.

Change-Id: I7e9ef4ddd82e2c2dd04ce09ec8d58a8829cfb64d
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>