mm: zcache: add core files

zcache is a backend for cleancache that takes file pages that are in the
process of being reclaimed and attempts to compress them and store them
in a RAM-based memory pool. This can result in a significant I/O reduction
if system is full with file pages and, in the case where decompressing
from RAM is faster than reading from the disk, can also improve workload
performance.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Patch-mainline: linux-mm @ 2013-08-06 11:36:14
[vinmenon@codeaurora.org: trivial merge conflict fixes, checkpatch fixes]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Change-Id: Id05bcc1946eee22b94a83adeb0134a3f6e357ef8