LU-10421 echo: use echo layer when finding stripe object

In echo_md_dir_stripe_choose(), find the stripe object using the echo
device rather than the down layer (mdd) device. mdd objects are not
equipped to be top layer objects and should not be found in this way.

Test-Parameters: trivial testlist=mds-survey
Lustre-change: https://review.whamcloud.com/31338


Lustre-commit: 6f60a282

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ibb396ae64b6d542c64697336d227e06163a0bb39
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/31556


Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>