LU-17810 dne: dir restripe without fixed hash flag

LU-17810 dne: dir restripe without fixed hash flag

Add an option to clear LMV_HASH_FIXED_FLAG
in dir re-striping, this restores an ability to convert a
striped dir with only one stripe into a plain dir.

HPE-bug-id: LUS-12321
Fixes: 4c2514f483 ("LU-14459 mdt: support fixed directory layout")
Signed-off-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: I7c119f435b19a446cb960cdc5a8ebec7e00ab0dc
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57117
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>