block: use printk_once

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Tim Waugh <tim@cyberelk.net>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>