LU-13417 test: use mkdir_on_mdt0() in replay-dual

LU-13417 test: use mkdir_on_mdt0() in replay-dual

Replace mkdir with mkdir_on_mdt0() in replay-dual.sh if directory
needs to be created on MDT0.

Lustre-change: https://review.whamcloud.com/43492
Lustre-commit: ce179e97767936ff76282fd06df063b386851fe7

Test-Parameters: trivial mdscount=2 mdtcount=4 testlist=replay-dual
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I9093e633412991571e18cb0ea264af013672bd8b
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44462
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>