LU-8130 ptlrpc: convert conn_hash to rhashtable

LU-8130 ptlrpc: convert conn_hash to rhashtable

Linux has a resizeable hashtable implementation in lib,
so we should use that instead of having one in libcfs.

This patch converts the ptlrpc conn_hash to use rhashtable.
In the process we gain lockless lookup.

As connections are never deleted until the hash table is destroyed,
there is no need to count the reference in the hash table.  There
is also no need to enable automatic_shrinking.

Linux-commit: ac2370ac2bc5215daf78546cd8d925510065bb7f

Change-Id: I576daf314c3ac31a58df02d731292e1e8bb408c6
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/32036
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>