LU-14366 mdt: lfs mkdir should return -EEXIST if exists

LU-14366 mdt: lfs mkdir should return -EEXIST if exists

'lfs setdirstripe' will try restripe if target exists, however
it's confusing to get -ENOTSUPP or -EALREADY for 'lfs mkdir', while
the latter invokes the same function as 'lfs setdirstripe'.

Pack MDS_OPEN_CREAT flag in request for 'lfs mkdir', and MDT won't
try restripe if it's set.

Add sanity 230s.

Lustre-change: https://review.whamcloud.com/41329
Lustre-commit: 65e3e4050ec5bb371c1c343fca49a605286a086e

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I7b7ed04ee0b150253ff4d13bbdf1fe847d8f577c
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/43428
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>