LU-8066 obdclass: move lu_ref to debugfs

LU-8066 obdclass: move lu_ref to debugfs

A special procfs file is created for lu_ref debugging. Lets move
this to debugfs where it belongs.

Also fixed a missed USE_LU_REF due to landing order as well as
a build fix.

Fixes: dfe2d225b86 ("LU-13799 clio: Implement real list splice")
Change-Id: I33646a87adfcabc5a5f214832953b2444e7aaf0a
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/44311
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>