LU-15553 test: mkdir_on_mdt0 in recovery-small.sh

LU-15553 test: mkdir_on_mdt0 in recovery-small.sh

Many subtests in recovery-small.sh requires test dir be created on
MDT0, replace mkdir with mkdir_on_mdt0.

Fixes: b9c4dc3c33 ("LU-14792 llite: enable filesystem-wide default LMV")

Lustre-change: https://review.whamcloud.com/51669
Lustre-commit: 3b0d2821845cf87ae7f03bf41ceae00237d94121

Test-Parameters: trivial
Test-Parameters: testlist=recovery-small,recovery-small,recovery-small
Test-Parameters: mdscount=2 mdtcount=4 testlist=recovery-small,recovery-small,recovery-small
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ibc37b2dd25bcd94794392f5ff8a79df2e7932dcc
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/55059
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>