lustre/lov: avoid subobj's coh_parent race

* during a file lov object initialization, we need protect the access
  and change of its subobj->coh_parent, since it could be another
  layout change race there, which makes an unreferenced lovsub obj in
  the site object hash table.

* dump lovsub objects in the site if the lovsub device reference > 0
  during its finalization phase.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Reviewed-on: http://review.whamcloud.com/6105
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-1480


Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>