LU-4825 utils: improve lfs_migrate usage message

LU-4825 utils: improve lfs_migrate usage message

Improve the warning message printed by lfs_migrate to make it match
the OST device names used in Lustre 2.x instead of 1.x device names.
Also reference the new max_create_count parameter instead of the old
deactivate command, which prevented OST objects from being deleted.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I114006afb93d8c8d78923a874f3b914202500c1e
Reviewed-on: https://review.whamcloud.com/25661
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Steve Guminski <stephenx.guminski@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>