LU-16227 utils: Add warning for lfs setdirstripe -D -i x,y,z

LU-16227 utils: Add warning for lfs setdirstripe -D -i x,y,z

Adjust setdirstripe to be more user friendly. The
use of "-D -i x,y,z" now returns a clear error
that this is creating a default striped directory
layout and that this is a bad idea, if it is not
accompanied by "-c N" that matches the number of
index values given.

Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Ic9f91853d4016bf0edfb3845ac9f1edafdf73d55
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49420
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>