staging: lustre: obd: limit lu_object cache

As the LU cache grows it can consume large enough chunks of
memory that ends up preventing buffers for other objects,
such as the OIs, from being cached and severely impacting
the performance for FID lookups. Limit the lu_object cache
to a maximum of lu_cache_nr objects.

NOTES:

* In order to be able to quickly determine the number of objects in
  the hash table the CFS_HASH_COUNTER flag is added.  This adds an
  atomic_inc/dec to the hash insert/remove paths but is not expected
  to have any measurable impact of performance.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5164
Reviewed-on: http://review.whamcloud.com/10237
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Isaac Huang <he.huang@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>

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