LU-8130 obd: convert obd uuid hash to rhashtable

LU-8130 obd: convert obd uuid hash to rhashtable

The rhashtable data type is a perfect fit for the
export uuid hash table, so use that instead of
cfs_hash (which will eventually be removed).

As rhashtable supports lookups and insertions in atomic
context, there is no need to drop a spinlock while
inserting a new entry, which simplifies code quite a bit.

Linux-commit: 4206c444e4a89dae9f67f08d9c29d58c37c960cd

Change-Id: Icadf64d572982409008a1ef4d23eb0fe1e3c8cd0
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://review.whamcloud.com/34429
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>