LU-744 obdclass: revise stats for cl_object cache

LU-744 obdclass: revise stats for cl_object cache

It turns out that atomic doesn't scale well for highly contented
cl_page stats. Enable it only when CONFIG_DEBUG_PAGESTATE_TRACKING
is turned on.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I0163f65557135759f100fddde70c7e3ee2e45aed
Reviewed-on: http://review.whamcloud.com/4519
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>