mtd: Clean refcounting with MTD_PARTITIONED_MASTER

The logic is way too convoluted, let's clean the kref_get/put section to
clarify what this block does when using CONFIG_MTD_PARTITIONED_MASTER:
- Iterate through all the parent mtd devices
- Grab a reference over them all but the master
- Only grab the master whith CONFIG_MTD_PARTITIONED_MASTER
Same logic must apply in the put path, otherwise it would be broken.

Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: Alexander Usyskin <alexander.usyskin@intel.com>
Cc: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Fixes: 19bfa9e ("mtd: use refcount to prevent corruption")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Alexander Usyskin <alexander.usyskin@intel.com>
Link: https://lore.kernel.org/linux-mtd/20230731090903.770277-1-miquel.raynal@bootlin.com

Author: miquelraynal

drivers/mtd/mtdcore.c

@@ -1247,14 +1247,15 @@ int __get_mtd_device(struct mtd_info *mtd)
 		return -ENODEV;
 	}
 
-	kref_get(&mtd->refcnt);
-
-	while (mtd->parent) {
-		if (IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER) || mtd->parent != master)
-			kref_get(&mtd->parent->refcnt);
+	while (mtd) {
+		if (mtd != master)
+			kref_get(&mtd->refcnt);
 		mtd = mtd->parent;
 	}
 
+	if (IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER))
+		kref_get(&master->refcnt);
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(__get_mtd_device);
@@ -1338,10 +1339,12 @@ void __put_mtd_device(struct mtd_info *mtd)
 {
 	struct mtd_info *master = mtd_get_master(mtd);
 
-	while (mtd != master) {
+	while (mtd) {
+		/* kref_put() can relese mtd, so keep a reference mtd->parent */
 		struct mtd_info *parent = mtd->parent;
 
-		kref_put(&mtd->refcnt, mtd_device_release);
+		if (mtd != master)
+			kref_put(&mtd->refcnt, mtd_device_release);
 		mtd = parent;
 	}