md/raid0: fix NULL pointer dereference in create_strip_zones() for dm-raid

Commit 2107457 ("md/raid0: Move queue limit setup before r0conf
initialization") dereference mddev->gendisk unconditionally, which is
NULL for dm-raid.

Fix this problem by reverting to old codes for dm-raid.

Link: https://lore.kernel.org/linux-raid/20251116021816.107648-1-yukuai@fnnas.com
Fixes: 2107457 ("md/raid0: Move queue limit setup before r0conf initialization")
Reported-and-tested-by: Changhui Zhong <czhong@redhat.com>
Closes: https://lore.kernel.org/all/CAGVVp+VqVnvGeneUoTbYvBv2cw6GwQRrR3B-iQ-_9rVfyumoKA@mail.gmail.com/
Signed-off-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Reviewed-by: Li Nan <linan122@huawei.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>

Author: hailan94

drivers/md/raid0.c

@@ -68,7 +68,10 @@ static int create_strip_zones(struct mddev *mddev, struct r0conf **private_conf)
 	struct strip_zone *zone;
 	int cnt;
 	struct r0conf *conf = kzalloc(sizeof(*conf), GFP_KERNEL);
-	unsigned int blksize = queue_logical_block_size(mddev->gendisk->queue);
+	unsigned int blksize = 512;
+
+	if (!mddev_is_dm(mddev))
+		blksize = queue_logical_block_size(mddev->gendisk->queue);
 
 	*private_conf = ERR_PTR(-ENOMEM);
 	if (!conf)
@@ -84,6 +87,10 @@ static int create_strip_zones(struct mddev *mddev, struct r0conf **private_conf)
 		sector_div(sectors, mddev->chunk_sectors);
 		rdev1->sectors = sectors * mddev->chunk_sectors;
 
+		if (mddev_is_dm(mddev))
+			blksize = max(blksize, queue_logical_block_size(
+				      rdev1->bdev->bd_disk->queue));
+
 		rdev_for_each(rdev2, mddev) {
 			pr_debug("md/raid0:%s:   comparing %pg(%llu)"
 				 " with %pg(%llu)\n",