LU-15553 test: mkdir_on_mdt0 in sanity-krb5

LU-15553 test: mkdir_on_mdt0 in sanity-krb5

test_8 in requires test dir created on MDT0, replace mkdir
with mkdir_on_mdt0. It's found by script:
grep -C 10 -n "do_facet.*SINGLEMDS" lustre/tests/*.sh | grep -w mkdir

Fixes: b9c4dc3c33 ("LU-14792 llite: enable filesystem-wide default LMV")
Test-Parameters: trivial mdscount=2 mdtcount=4 testlist=sanity-krb5,sanity-krb5,sanity-krb5
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I09b1aec95bff84622accea91650887dffc1245f3
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51653
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>