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

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I0537564ba544ed06be42ba243606a884a1290f20
Reviewed-on: https://review.whamcloud.com/33616
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>