LU-8130 lov: convert lo[v|d]_pool to use rhashtable

The pools hashtable can be implemented using
the rhashtable implementation in lib.
This has the benefit that lookups are lock-free.

We need to use kfree_rcu() to free a pool so
that a lookup racing with a deletion will not access
freed memory.

rhashtable has no combined lookup-and-delete interface,
but as the lookup is lockless and the chains are short,
this brings little cost.  Even if a lookup finds a pool,
we must be prepared for the delete to fail to find it,
as we might race with another thread doing a delete.

We use atomic_inc_not_zero() after finding a pool in the
hash table and if that fails, we must have raced with a
deletion, so we treat the lookup as a failure.

Use hashlen_string() rather than a hand-crafted hash
function.
Note that the pool_name, and the search key, are
guaranteed to be nul terminated.

Based on

Linux-commit: 055ed193b190edac539f37a66699b02eae3a19a9

with the port of server side pool handling to rhashtables.

Change-Id: Ia5b4cbbd17515ea43a473e91719b3665f46b0d0a
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/32662
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>