LU-8616 dne: allow mkdir with specific MDTs

LU-8616 dne: allow mkdir with specific MDTs

Silimiar to create file on specific OSTs, allow 'lfs mkdir' to mkdir
on specific MDTs. This is achieved by allowing 'lfs mkdir -i' specify
multiple MDTs, and adding LMV_USER_MAGIC_SPECIFIC.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I3876707103465d1659afc80914ed6f9b58da25eb
Reviewed-on: https://review.whamcloud.com/30566
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>