mm: cleancache: update to match akpm frontswap feedback

This is the fifth patch of six in the frontswap series; changes made to
frontswap due to feedback from akpm/others must also be made to cleancache
for consistency.  These are: (1) change sysfs to debugfs; (2) change
use of the term "flush" to "invalidate".  Note that some changes
for (2) are deferred to the next patch in this patchset in order to
coordinate with simultaneous required driver changes.

[v10: no change]
[v9: akpm@linux-foundation.org: sysfs->debugfs; no longer need Doc/ABI file]
[v9: akpm@linux-foundation.org: change "flush" to "invalidate", part 2]

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Reviewed-by: Konrad Wilk <konrad.wilk@oracle.com>
Cc: Jan Beulich <JBeulich@novell.com>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Rik Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
modified for Linux 3.1.x kernel from LKML

Signed-off-by: Paul Reioux <reioux@gmail.com>

Conflicts:
	mm/cleancache.c