btrfs: use btrfs_read_node_slot in btrfs_qgroup_trace_subtree

	level = root_level;

	while (level >= 0) {

		if (path->nodes[level] == NULL) {

			struct btrfs_key first_key;

			int parent_slot;

			u64 child_gen;

			u64 child_bytenr;

			/*

			 * We need to get child blockptr/gen from parent before

			 * we can read it.

			 * We need to get child blockptr from parent before we

			 * can read it.

			  */

			eb = path->nodes[level + 1];

			parent_slot = path->slots[level + 1];

			child_bytenr = btrfs_node_blockptr(eb, parent_slot);

			child_gen = btrfs_node_ptr_generation(eb, parent_slot);

			btrfs_node_key_to_cpu(eb, &first_key, parent_slot);

			eb = read_tree_block(fs_info, child_bytenr, child_gen,

					     level, &first_key);

			eb = btrfs_read_node_slot(eb, parent_slot);

			if (IS_ERR(eb)) {

				ret = PTR_ERR(eb);

				goto out;

			} else if (!extent_buffer_uptodate(eb)) {

				free_extent_buffer(eb);

				ret = -EIO;

				goto out;

			}

			path->nodes[level] = eb;