md/bitmap: resume array on backlog_store() error path

backlog_store() suspends the array before checking whether a write-mostly
device exists. If no such device exists, the error path only unlocks
reconfig_mutex and leaves the array suspended, blocking subsequent I/O.

Use mddev_unlock_and_resume() to release both states.

Fixes: 58226942ad ("md: use new apis to suspend array before mddev_create/destroy_serial_pool")

Signed-off-by: Chen Cheng <chencheng@fnnas.com>
Reviewed-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/20260718034236.4119093-1-chencheng@fnnas.com
Signed-off-by: Yu Kuai <yukuai@fygo.io>

drivers/md/md-bitmap.c

@@ -2857,7 +2857,7 @@ backlog_store(struct mddev *mddev, const char *buf, size_t len)
 	if (!has_write_mostly) {
 		pr_warn_ratelimited("%s: can't set backlog, no write mostly device available\n",
 				    mdname(mddev));
-		mddev_unlock(mddev);
+		mddev_unlock_and_resume(mddev);
 		return -EINVAL;
 	}