LU-19070 dne: dir migrate allowed only for root

LU-19070 dne: dir migrate allowed only for root

Current implemetation of lfs migrate -m
relies on setxttr(, "trusted.lmv", ) which is
allowed only for users with CAP_SYS_ADMIN capability.
Adding the same check to ll_migrate() will prevent
incomplete migrations from a non-root user.
Add error reporting to cb_migrate_mdt_fini().

Fixes: 0a83d948f3 ("LU-4684 migrate: shrink dir layout after migration")
Fixes: 2dae2b8ffb ("LU-8777 mdt: add parameter to disable remote/striped dir")
HPE-bug-id: LUS-12895
Signed-off-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: I58d417b64e2b634d76e4ad38685deb21d9ce8a86
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59474
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>