LU-15830 mdt: mkdir to lookup target name

LU-15830 mdt: mkdir to lookup target name

distributed mkdir should lookup the target name to
avoid rollback as much as possible as the latter is
very expensive due to llog re-initialization.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: If28760e0afb804dca11e1e7501e0a53ff9067ca1
Reviewed-on: https://review.whamcloud.com/47226
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>