ubi: fastmap: Free fastmap next anchor peb during detach

ubi_wl_entry related with the fm_next_anchor PEB is not freed during
detach, which causes a memory leak.
Don't forget to release fm_next_anchor PEB while detaching ubi from
mtd when CONFIG_MTD_UBI_FASTMAP is enabled.
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Fixes: 4b68bf9a ("ubi: Select fastmap anchor PEBs considering...")
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/fastmap-wl.c

@@ -381,6 +381,11 @@ static void ubi_fastmap_close(struct ubi_device *ubi)
 		ubi->fm_anchor = NULL;
 	}
 
+	if (ubi->fm_next_anchor) {
+		return_unused_peb(ubi, ubi->fm_next_anchor);
+		ubi->fm_next_anchor = NULL;
+	}
+
 	if (ubi->fm) {
 		for (i = 0; i < ubi->fm->used_blocks; i++)
 			kfree(ubi->fm->e[i]);