LU-11739 lod: don't inherit default layout from root directory

LU-11739 lod: don't inherit default layout from root directory

There is no need to inherit the default directory layout from
the root directory when subdirectories are created therein.
This consumes xattr space on the subdirectories, and makes it
more complex to change the filesystem default layout in the future.

This patch fixes the above issue in lod_ah_init() to check if
the parent directory is the root directory and not copy
the default layout xattr to the new subdirectory.

Lustre-change: https://review.whamcloud.com/33956
Lustre-commit: 0a988cae95f99fee1a9c0d489ce00d0954d2a68e

Lustre-change: https://review.whamcloud.com/34175
Lustre-commit: ad1a74527f0ec59510bfa124b8280617a2b93840

Change-Id: Ie0d286785bdbcd73e2ae60b429e66d5d54b44eef
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34135
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>