mtd: Create partname and partid debug files for child MTDs

Partname and partid are set by the upper driver (spi-nor) on the master
MTD. If this MTD is partitioned and CONFIG_MTD_PARTITIONED_MASTER is
disabled, the master MTD is not instantiated and partname and partid
aren't available to the userspace.

Always read the partname and partid from the master MTD, they describe
the HW, which can't differ between master and its children.
Signed-off-by: Petr Malat <oss@malat.biz>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210430065057.32018-1-oss@malat.biz

drivers/mtd/mtdcore.c

@@ -360,6 +360,7 @@ static struct dentry *dfs_dir_mtd;
 
 static void mtd_debugfs_populate(struct mtd_info *mtd)
 {
+	struct mtd_info *master = mtd_get_master(mtd);
 	struct device *dev = &mtd->dev;
 	struct dentry *root;
 
@@ -369,12 +370,12 @@ static void mtd_debugfs_populate(struct mtd_info *mtd)
 	root = debugfs_create_dir(dev_name(dev), dfs_dir_mtd);
 	mtd->dbg.dfs_dir = root;
 
-	if (mtd->dbg.partid)
-		debugfs_create_file("partid", 0400, root, mtd,
+	if (master->dbg.partid)
+		debugfs_create_file("partid", 0400, root, master,
 				    &mtd_partid_debug_fops);
 
-	if (mtd->dbg.partname)
-		debugfs_create_file("partname", 0400, root, mtd,
+	if (master->dbg.partname)
+		debugfs_create_file("partname", 0400, root, master,
 				    &mtd_partname_debug_fops);
 }