lowmemorykiller: add zcache awareness

zcache stores compressed file pages which should be
considered as reclaimable pages by lowmemorykiller
in calculating values to be compared against minfree.

Change-Id: Ia3e08bc14ba61c0a45ed54ba5cd525717a572060
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>