md: warn about updating super block failure

Many personalities will handle IO error from daemon thread(like raid1d,
raid10d, raid5d), and sb will require to be clean before hanlding these
failed IO. However update sb can fail, for example array is broken by
IO failure, or user config sysfs api array_state.

This patch adds warning if updating sb failed first, in case this will
be related to IO hang.

Link: https://lore.kernel.org/linux-raid/20251117085557.770572-2-yukuai@fnnas.com
Signed-off-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Li Nan <linan122@huawei.com>

Author: hailan94

drivers/md/md.c

@@ -2788,6 +2788,7 @@ void md_update_sb(struct mddev *mddev, int force_change)
 	if (!md_is_rdwr(mddev)) {
 		if (force_change)
 			set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);
+		pr_err("%s: can't update sb for read-only array %s\n", __func__, mdname(mddev));
 		return;
 	}