LU-11213 uapi: change "space" hash type to hash flag

LU-11213 uapi: change "space" hash type to hash flag

Change LMV_HASH_TYPE_SPACE to LMV_HASH_FLAG_SPACE to make it flexible
in directory layout inheritance in the future. But it's still exposed
to user as hash type "space" in "lfs setdirstripe" command to make
it easy to understand.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ifa302204ed62dff8cc9d12fdc1f9ea86f8491d40
Reviewed-on: https://review.whamcloud.com/35318
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>