Btrfs: send, avoid unnecessary inode item lookup in the btree

Regardless of whether the caller is interested or not in knowing the inode's
generation (dir_gen != NULL), get_first_ref always does a btree lookup to get
the inode item. Avoid this useless lookup if dir_gen parameter is NULL (which
is in some cases).
Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/send.c

@@ -1693,10 +1693,12 @@ static int get_first_ref(struct btrfs_root *root, u64 ino,
 		goto out;
 	btrfs_release_path(path);
 
-	ret = get_inode_info(root, parent_dir, NULL, dir_gen, NULL, NULL,
-			NULL, NULL);
+	if (dir_gen) {
+		ret = get_inode_info(root, parent_dir, NULL, dir_gen, NULL,
+				     NULL, NULL, NULL);
 		if (ret < 0)
 			goto out;
+	}
 
 	*dir = parent_dir;
 
@@ -1712,13 +1714,12 @@ static int is_first_ref(struct btrfs_root *root,
 	int ret;
 	struct fs_path *tmp_name;
 	u64 tmp_dir;
-	u64 tmp_dir_gen;
 
 	tmp_name = fs_path_alloc();
 	if (!tmp_name)
 		return -ENOMEM;
 
-	ret = get_first_ref(root, ino, &tmp_dir, &tmp_dir_gen, tmp_name);
+	ret = get_first_ref(root, ino, &tmp_dir, NULL, tmp_name);
 	if (ret < 0)
 		goto out;