LU-14459 mdt: support fixed directory layout

LU-14459 mdt: support fixed directory layout

User may not want directories split automatically in some cases:
*.directory migrated.
* directory restriped.

To support this, an LMV flag LMV_HASH_FLAG_FIXED is added, and it will
be set on migrated/restriped directories. NB, if directory is migrated
or restriped to a one-stripe directory, it won't be transformed into a
plain directory, because this flag needs to be kept.

Update sanity 230q.

Lustre-change: https://review.whamcloud.com/43291
Lustre-commit: 4c2514f4832801374092f3a48c755248af345566

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Icd12b2aa34d391e32c3323a8b9c24449ea3e3d0e
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44459
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>