LU-13417 test: add mkdir_on_mdt0()

LU-13417 test: add mkdir_on_mdt0()

Once default LMV is set on ROOT, and default stripe offset is "-1",
mkdir may not create directory on MDT0, but it's a premise for many
tests. Add a function mkdir_on_mdt0() to create directory on MDT0
by "lfs mkdir -i 0".

Replace mkdir with mkdir_on_mdt0() for such tests in sanity.sh and
sanityn.sh.

Test-Parameters: trivial testlist=sanityn
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I6155d036e6b28153d0bdbdbc01088bd68ee9e0af
Reviewed-on: https://review.whamcloud.com/43489
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>