mtd: core: Fix refcounting for unpartitioned MTDs

stable inclusion
from stable-5.10.4
commit d5e3fc275ea1f0f67f3b713cc866eb3248ae4080
bugzilla: 46903

--------------------------------

commit 1ca71415 upstream.

Apply changes to usecount also to the master partition.
Otherwise we have no refcounting at all if an MTD has no partitions.

Cc: stable@vger.kernel.org
Fixes: 46b5889c ("mtd: implement proper partition handling")
Signed-off-by: NRichard Weinberger <richard@nod.at>
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201206202220.27290-1-richard@nod.atSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>

drivers/mtd/mtdcore.c

@@ -993,6 +993,8 @@ int __get_mtd_device(struct mtd_info *mtd)
 		}
 	}
 
+	master->usecount++;
+
 	while (mtd->parent) {
 		mtd->usecount++;
 		mtd = mtd->parent;
@@ -1059,6 +1061,8 @@ void __put_mtd_device(struct mtd_info *mtd)
 		mtd = mtd->parent;
 	}
 
+	master->usecount--;
+
 	if (master->_put_device)
 		master->_put_device(master);