LU-11213 dne: add new dir hash type "space"

Add a new hash type "space", if this is set on default LMV of
a directory, its subdirs will be created on all MDTs with
balanced space usage.

* new hash type LMV_HASH_TYPE_SPACE.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I8edf38f94e24965b1cffb21253c3be0eef68c707
Reviewed-on: https://review.whamcloud.com/34358
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>