LU-8130 obd: convert obd_nid_hash to rhashtable

LU-8130 obd: convert obd_nid_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 struct obd_export obd_nid_hash to use
rhashtable. In the process we gain lockless lookup which should
improve performance. For the nid hash we use rhltable since the
mapping can be many exports to a NID key.

Signed-off-by: James Simmons <jsimmons@infradead.org>
Change-Id: I45154ceb48336b20161f771d986d8fe7333b9849
Reviewed-on: https://review.whamcloud.com/33518
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>