block: sed-opal: rename next to execute_steps

As the function is responsible for executing the individual steps supplied
in the steps argument, execute_steps is a more descriptive name than the
rather generic next.

Signed-off-by: David Kozub <zub@linux.fjfi.cvut.cz>
Reviewed-by: Scott Bauer <sbauer@plzdonthack.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>