LU-15553 test: mkdir_on_mdt0 in conf-sanity.sh

LU-15553 test: mkdir_on_mdt0 in conf-sanity.sh

Change mkdir to mkdir_on_mdt0 in several conf-sanity.sh sub tests.

Fixes: b9c4dc3c33 ("LU-14792 llite: enable filesystem-wide default LMV")
Test-Parameters: trivial testlist=conf-sanity mdtcount=4
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I5ace9df10e725802ba502ca20c60afc708b857cc
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56539
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>