md/raid10: fix null ptr dereference in raid10_size()

In raid10_run() if raid10_set_queue_limits() succeed, the return value
is set to zero, and if following procedures failed raid10_run() will
return zero while mddev->private is still NULL, causing null ptr
dereference in raid10_size().

Fix the problem by only overwrite the return value if
raid10_set_queue_limits() failed.

Fixes: 3d8466b ("md/raid10: use the atomic queue limit update APIs")
Cc: stable@vger.kernel.org
Reported-and-tested-by: ValdikSS <iam@valdikss.org.ru>
Closes: https://lore.kernel.org/all/0dd96820-fe52-4841-bc58-dbf14d6bfcc8@valdikss.org.ru/
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20241009014914.1682037-1-yukuai1@huaweicloud.com
Signed-off-by: Song Liu <song@kernel.org>

Author: YuKuai-huawei

drivers/md/raid10.c

@@ -4061,9 +4061,12 @@ static int raid10_run(struct mddev *mddev)
 	}
 
 	if (!mddev_is_dm(conf->mddev)) {
-		ret = raid10_set_queue_limits(mddev);
-		if (ret)
+		int err = raid10_set_queue_limits(mddev);
+
+		if (err) {
+			ret = err;
 			goto out_free_conf;
+		}
 	}
 
 	/* need to check that every block has at least one working mirror */