offtab_get
uint64_t offtab_get(struct offtab *, uint32_t);
const uint64_t first_offset = offtab_get(&S->offtab, 0);
const uint64_t offset = offtab_get(&S->offtab, blkno);
const uint64_t offset = offtab_get(&S->offtab, nblkno);
uint64_t last = offtab_get(&offtab, 0);
const uint64_t end = offtab_get(&offtab, (blkno + 1));