zcache: The last of the atomic reads has now an accessory function.

And now we can move the code ([inc|dec]_zcache_[*]) to their own file
with a header to make them nops or feed in debugfs.

Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>