staging/lustre/obdclass: Eliminate hash bucket scans in lu_cache_shrink

The lu_cache_shrink slab shrinker is too slow, accounting for > 90% of
the time spent in shrink_slab when allocating huge pages. Most of its
time is spent iterating over the buckets in each site's object hash
table to compute the number of freeable objects. This iteration is
eliminated by adding an lru length count to the lu_site struct. A
percpu counter is used to maintain the lru length, so that the
lu_site does not need to be locked when an object is accessed through
the hash table. A counter is updated whenever an object is added to
or deleted from any of the hash table buckets. The number of freeable
objects is the sum of the counter values across all cpus.

Signed-off-by: Ann Koehler <amk@cray.com>
Reviewed-on: http://review.whamcloud.com/14066
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6365


Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>