btrfs_search_extent_mapping
existing = btrfs_search_extent_mapping(&inode->extent_tree, start, len);
struct extent_map *btrfs_search_extent_mapping(struct extent_map_tree *tree,
em = btrfs_search_extent_mapping(em_tree, start, num_bytes);
em = btrfs_search_extent_mapping(em_tree, 0, 0);
em = btrfs_search_extent_mapping(em_tree, ordered->file_offset,