LU-6566 mdt: optimize sub_stripe lock/unlock

LU-6566 mdt: optimize sub_stripe lock/unlock

In lod_object_unlock, since it already know the lock
handle, it can release the lock directly, instead of
load and holding the sub-stripe object.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Id9267f3619b036ce943a1df6ae163ea49a0fbc25
Reviewed-on: http://review.whamcloud.com/14679
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>