LU-16500 utils: set default ost index for lfs migrate

LU-16500 utils: set default ost index for lfs migrate

Running "lfs migrate <file>" without any SETSTRIPE arguments
to balance space usage keeps the PFL file layout, but preserves
the OST selection exactly, which makes the migration virtually
useless for space balancing.

This patch fixes the above issue by clearing the specific
OST indices from the source layout before using the layout to
create the volatile file in lfs_migrate().

Signed-off-by: Jian Yu <yujian@whamcloud.com>
Change-Id: I82e1dc0a11fdda7d555df994cf4e5f6e3dbdcb5c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49819
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Zhenyu Xu <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>