Btrfs: Fix memory leak at umount

fs_info, which is allocated in open_ctree(), should be freed
in close_ctree().
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

fs/btrfs/disk-io.c

@@ -2513,6 +2513,8 @@ int close_ctree(struct btrfs_root *root)
 	kfree(fs_info->chunk_root);
 	kfree(fs_info->dev_root);
 	kfree(fs_info->csum_root);
+	kfree(fs_info);
+
 	return 0;
 }