Fewer uses of size_t in sparse.c

Fewer uses of size_t in sparse.c

* src/sparse.c (struct tar_sparse_optab, dump_zeros)
(tar_sparse_dump_region, tar_sparse_extract_region)
(zero_block_p, sparse_add_map, sparse_dump_region)
(sparse_extract_region, sparse_dump_file, sparse_extract_file)
(check_data_region, sparse_diff_file, oldgnu_get_sparse_info)
(oldgnu_store_sparse_info, oldgnu_dump_header)
(star_get_sparse_info, pax_dump_header_0):
Prefer idx_t to size_t.