LU-744 obdclass: revise percpu stats of lprocfs_stats

LU-744 obdclass: revise percpu stats of lprocfs_stats

Atomic doesn't scale well and there is an atomic counter to remember
if updating stats is on going. This will introduce performance
problem if the percpu stats is used in a highly contended code.

I'll remove that atomic counter in this patch, this is okay as percpu
counter is not accurate essentially.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I5c7a094b78ab53798266d48d56f3a16e7c7d436f
Reviewed-on: http://review.whamcloud.com/4472
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>