staging: lustre: osc: revise unstable pages accounting

staging: lustre: 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 controlled by
   llite.*.unstable_stats.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4841
Reviewed-on: http://review.whamcloud.com/10003
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>