LU-12055 lod: ignore root layout if parent's one includes pool

LU-12055 lod: ignore root layout if parent's one includes pool

When parent directory provides striping information which includes a
pool, don't inherit the stripe index from the root directory striping.
Otherwise, pool config and root layout mismatch leads to create
failure.

Test illustrating the issue is added.

Fix for sanity.sh:test_255c: the test assumes that test dir has
stripe count set to 1. Specify it explicitly to avoid using stripe
count of root directory which might be not 1.

Signed-off-by: Vladimir Saveliev <c17830@cray.com>
Cray-bug-id: LUS-7093
Change-Id: Ib0f61edf4973bcb260dad617c27d1c1729223fa0
Reviewed-on: https://review.whamcloud.com/34386
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>