DFS/jffs2: fix bug when umount jffs2

found by haitao5198
clear mount parition device talbe when jffs2 mount falied

components/dfs/filesystems/jffs2/dfs_jffs2.c

@@ -178,6 +178,7 @@ static int dfs_jffs2_mount(struct dfs_filesystem* fs,
 	result = jffs2_mount(NULL, mte);
 	if (result != 0)
 	{	
+		device_partition[index].dev = NULL;
 		return jffs2_result_to_dfs(result);
 	}
 	/* save this pointer */