LU-8130 lu_object: convert lu_object cache to rhashtable

LU-8130 lu_object: convert lu_object cache to rhashtable

The lu_object cache is a little more complex than the other lustre
hash tables for two reasons.
1/ there is a debugfs file which displays the contents of the cache,
  so we need to use rhashtable_walk in a way that works for seq_file.

2/ There is a (sharded) lru list for objects which are no longer
   referenced, so finding an object needs to consider races with the
   lru as well as with the hash table.

The debugfs file already manages walking the libcfs hash table keeping
a current-position in the private data.  We can fairly easily convert
that to a struct rhashtable_iter.  The debugfs file actually reports
pages, and there are multiple pages per hashtable object.  So as well
as rhashtable_iter, we need the current page index.

For the double-locking, the current code uses direct-access to the
bucket locks that libcfs_hash provides.  rhashtable doesn't provide
that access - callers must provide their own locking or use rcu
techniques.

The lsb_waitq.lock is still used to manage the lru list, but with
this patch it is no longer nested *inside* the hashtable locks, but
instead is outside.  It is used to protect an object with a refcount
of zero.

When purging old objects from an lru, we first set
LU_OBJECT_HEARD_BANSHEE while holding the lsb_waitq.lock,
then remove all the entries from the hashtable separately.

When removing the last reference from an object, we first take the
lsb_waitq.lock, then decrement the reference and add to the lru list
or discard it setting LU_OBJECT_UNHASHED.

When we find an object in the hashtable with a refcount of zero, we
take the corresponding lsb_waitq.lock and check that neither
LU_OBJECT_HEARD_BANSHEE or LU_OBJECT_UNHASHED is set.  If neither is,
we can safely increment the refcount.  If either are, the object is
gone.

This way, we only ever manipulate an object with a refcount of zero
while holding the lsb_waitq.lock.

As there is nothing to stop us using the resizing capabilities of
rhashtable, the code to try to guess the perfect hash size has been
removed.

Also: the "is_dying" variable in lu_object_put() is racey - the value
could change the moment it is sampled.  It is also not needed as it is
only used to avoid a wakeup, which is not particularly expensive.
In the same code as comment says that 'top' could not be accessed, but
the code then immediately accesses 'top' to calculate 'bkt'.
So move the initialization of 'bkt' to before 'top' becomes unsafe.

Also: Change "wake_up_all()" to "wake_up()".  wake_up_all() is only
relevant when an exclusive wait is used.

Moving from the libcfs hashtable to rhashtable also gives the
benefit of a very large performance boost.

Before patch:

SUMMARY rate: (of 5 iterations)
   Operation                      Max            Min           Mean        Std Dev
   ---------                      ---            ---           ----        -------
   Directory creation:      12036.610      11091.880      11452.978        318.829
   Directory stat:          25871.734      24232.310      24935.661        574.996
   Directory removal:       12698.769      12239.685      12491.008        149.149
   File creation:           11722.036      11673.961      11692.157         15.966
   File stat:               62304.540      58237.124      60282.003       1479.103
   File read:               24204.811      23889.091      24048.577        110.245
   File removal:             9412.930       9111.828       9217.546        120.894
   Tree creation:            3515.536       3195.627       3442.609        123.792
   Tree removal:              433.917        418.935        428.038          5.545

After patch:

SUMMARY rate: (of 5 iterations)
   Operation                      Max            Min           Mean        Std Dev
   ---------                      ---            ---           ----        -------
   Directory creation:      11873.308        303.626       9371.860       4539.539
   Directory stat:          31116.512      30190.574      30568.091        335.545
   Directory removal:       13082.121      12645.228      12943.239        157.695
   File creation:           12607.135      12293.319      12466.647        138.307
   File stat:              124419.347     105240.996     116919.977       7847.165
   File read:               39707.270      36295.477      38266.011       1328.857
   File removal:             9614.333       9273.931       9477.299        140.201
   Tree creation:            3572.602       3017.580       3339.547        207.061
   Tree removal:              487.687          0.004        282.188        230.659

Change-Id: I618dc2e2da003c240a887126f600e7eac5df951c
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/36707
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>