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

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
Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ibb396ae64b6d542c64697336d227e06163a0bb39
Reviewed-on: https://review.whamcloud.com/31338
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>