null_blk: remove an unused variable assignment in null_add_dev

Fix up the recent blk_alloc_disk conversion.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210614060231.3965278-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -1849,7 +1849,6 @@ static int null_add_dev(struct nullb_device *dev)
 		if (!null_setup_fault())
 			goto out_cleanup_tags;
 
-		rv = -ENOMEM;
 		nullb->tag_set->timeout = 5 * HZ;
 		nullb->disk = blk_mq_alloc_disk(nullb->tag_set, nullb);
 		if (IS_ERR(nullb->disk)) {