LU-4841 osc: revise unstable pages accounting

LU-4841 osc: revise unstable pages accounting

A few changes are made in this patch for unstable pages tracking:

1. Remove kernel NFS unstable pages tracking because it killed
   performance
2. Track unstable pages as part of LRU cache. Otherwise Lustre
   can use much more memory than max_cached_mb
3. Remove obd_unstable_pages tracking to avoid using global
   atomic counter
4. Make unstable pages track optional. Tracking unstable pages is
   turned off by default, and can be controled by
   llite.*.unstable_stats.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I3bfae9376aac2e2c38539577d9db29a375bc95af
Reviewed-on: http://review.whamcloud.com/10003
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>