LU-8990 lod: put root at cleanup

LU-8990 lod: put root at cleanup

'lod_md_root' was put at precleanup, but soak test shows there exists
race, and some ongoing request may re-initialize it, move this put
to cleanup.

Also add debug code to dump remaining objects if lod device is still
referenced at lod_device_free().

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I6f1ab0ba149ccf95279c1182c90a5588607ad8fa
Reviewed-on: https://review.whamcloud.com/31143
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>