LU-11104 mdt: rename may cause deadlock

LU-11104 mdt: rename may cause deadlock

In rename locking, there are two situations we need to lock target
parent before source parent:
1. source parent is subdir of target parent.
2. source and target parents are both stripes of the same directory,
   and stripe index of source parent is after that of target parent.

But the check for the second situation is missing, which may cause
deadlock if another thread is taking stripe locks of their parent.

Cleanup mdd_is_subdir().

Add sanityn.sh test_81b.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: Ib96fb7b286e7dfdea868ef2fa4919f8d3f1567f9
Reviewed-on: https://review.whamcloud.com/32701
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>