LU-930 utils: fix lfs setstripe option description

LU-930 utils: fix lfs setstripe option description

Document the "-o mode" for the file create mode in the usage
message instead of the deprecated "-m mode" for setdirstripe.

Fixes: fdad38781ccc ("LU-11376 lmv: new foreign LMV format")
Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I49259c06be57e234083c3d0f981ab674931b25d2
Reviewed-on: https://review.whamcloud.com/37730
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Joseph Gmitter <jgmitter@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>