LU-6570 tests: fix 40a in sanityn.sh

LU-6570 tests: fix 40a in sanityn.sh

In sanityn.sh 40a, it should cache the parent lookup LOCK
Before check parallel directory lock, otherwise parent
revalidation might cause lock conflicts on MDT.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Ib4c7c667d4e1dd821df96497e06459db0d5c33f9
Reviewed-on: http://review.whamcloud.com/15366
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>