btrfs: use btrfs_read_node_slot in qgroup_trace_new_subtree_blocks

We're open-coding btrfs_read_node_slot() here, replace with the helper.
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -2052,10 +2052,8 @@ static int qgroup_trace_new_subtree_blocks(struct btrfs_trans_handle* trans,
 
 	/* Read the tree block if needed */
 	if (dst_path->nodes[cur_level] == NULL) {
-		struct btrfs_key first_key;
 		int parent_slot;
 		u64 child_gen;
-		u64 child_bytenr;
 
 		/*
 		 * dst_path->nodes[root_level] must be initialized before
@@ -2074,23 +2072,16 @@ static int qgroup_trace_new_subtree_blocks(struct btrfs_trans_handle* trans,
 		  */
 		eb = dst_path->nodes[cur_level + 1];
 		parent_slot = dst_path->slots[cur_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);
 
 		/* This node is old, no need to trace */
 		if (child_gen < last_snapshot)
 			goto out;
 
-		eb = read_tree_block(fs_info, child_bytenr, child_gen,
-				     cur_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;
 		}
 
 		dst_path->nodes[cur_level] = eb;