mm: zcache: clear zcache when low on file pages

When files pages are very low, it is better to clear off
zcache pages, since the freed memory can be used to sustain
an application in foreground. Moreover when file pages
are too low, we don't gain much by holding a few zcache
pages.

Change-Id: I88dd295d24b7de18fb3bc0788e0baeb6bfdb2f6d
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>