mm: zcache: shrink zcache on memory pressure

When file pages drops down, at a point zcache pages
can be more than the file pages, and even further when
we have to reclaim the maximum number of file pages
possible to launch an application, we need a way to
reclaim even the zcache pages at least to an extend
which makes it match with the number of file pages.
This can help in better foreground headroom.

Change-Id: I481bfb9961ed5cee47ebeae08eb910bb269b644c
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>