block/sed: Fix opal user range check and unused variables

Fixes check that the opal user is within the range, and cleans up unused
method variables.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Reviewed-by: Scott Bauer <scott.bauer@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>