Fewer uses of size_t in extract.c

Fewer uses of size_t in extract.c

* src/extract.c (struct delayed_set_stat, struct delayed_link)
(delay_set_stat, apply_nonancestor_delayed_set_stat)
(extract_file): Prefer idx_t to size_t.
(struct delayed_set_stat): Remove unused member xattr_map_size.