staging/zcache: Fix/improve zcache writeback code, tie to a config option

It was observed by Andrea Arcangeli in 2011 that zcache can get "full"
and there must be some way for compressed swap pages to be (uncompressed
and then) sent through to the backing swap disk.  A prototype of this
functionality, called "unuse", was added in 2012 as part of a major update
to zcache (aka "zcache2"), but was left unfinished due to the unfortunate
temporary fork of zcache.

This earlier version of the code had an unresolved memory leak
and was anyway dependent on not-yet-upstream frontswap and mm changes.
The code was meanwhile adapted by Seth Jennings for similar
functionality in zswap (which he calls "flush").  Seth also made some
clever simplifications which are herein ported back to zcache.  As a
result of those simplifications, the frontswap changes are no longer
necessary, but a slightly different (and simpler) set of mm changes are
still required [1].  The memory leak is also fixed.

Due to feedback from akpm in a zswap thread, this functionality in zcache
has now been renamed from "unuse" to "writeback".

Although this zcache writeback code now works, there are open questions
as how best to handle the policy that drives it.  As a result, this
patch also ties writeback to a new config option.  And, since the
code still depends on not-yet-upstreamed mm patches, to avoid build
problems, the config option added by this patch temporarily depends
on "BROKEN"; this config dependency can be removed in trees that
contain the necessary mm patches.

[1] https://lkml.org/lkml/2013/1/29/540/ https://lkml.org/lkml/2013/1/29/539/

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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