LU-6998 mdt: do not lock stripe0 object

LU-6998 mdt: do not lock stripe0 object

Do not lock stripe 0 for migrating object in
mdt_lock_slaves(), because stripe0 and master
object are the same object, which has been locked
before mdt_lock_slaves().

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I8322061b9b8d310a17fdd6bd8265a3781b14dc9d
Reviewed-on: http://review.whamcloud.com/15973
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>