LU-5164 osd: Limit lu_object cache

For OSDs like ZFS to perform optimally it's import that they be allowed
to manage their own cache.  This maximizes the likelyhood that the ARC
will prefetch and cache the right buffers.  In the existing ZFS OSD
code a cached LU object pins buffers in the ARC preventing them from
being dropped.  As the LU cache grows it can consume the entire ARC
preventing buffers for other objects, such as the OIs, from being
cached and severely impacting the performance for FID lookups.

By default this patch will only limit the LU cache for ZFS OSDs.

NOTES:

* Setting LU_CACHE_NR_ZFS_LIMIT to 0 results in an LBUG on the MDS.
  This may be because an object is being used without a reference.
  Setting a minimum value of 256 was arbitrary, ideally we would
  set this value to 0.

* 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>
Change-Id: Ia64838d50395f1d22e558631adbfa39d44e91606
Patch-Set: 3
Reviewed-on: http://review.whamcloud.com/10237
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
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>