LU-11213 lmv: mkdir with balanced space usage

LU-11213 lmv: mkdir with balanced space usage

If a plain directory default LMV hash type is "space", create
subdirs on all MDTs with balanced space usage:
* client mkdir allocate FID on MDT with balanced space usage
  (space QoS code is in next patch).
* MDT allows mkdir on different MDT with its parent if it has
  "space" hash type in default LMV, this is normally rejected
  because mkdir shouldn't create remote directory.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I284e21f334c07462211be4c8e38e965722d1e8a8
Reviewed-on: https://review.whamcloud.com/34360
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>