offtab_prepare_get
bool offtab_prepare_get(struct offtab *, uint32_t);
if (!offtab_prepare_get(&S->offtab, 0))
if (!offtab_prepare_get(&S->offtab, blkno))
if (!offtab_prepare_get(&S->offtab, nblkno))
(void)offtab_prepare_get(&offtab, 0);
(void)offtab_prepare_get(&offtab, (blkno + 1));