sm_ll_dec
int sm_ll_dec(struct ll_disk *ll, dm_block_t b, dm_block_t e, int32_t *nr_allocations);
r = sm_ll_dec(&smd->ll, b, e, &nr_allocations);
r = sm_ll_dec(&smm->ll, op->b, op->e, &nr_allocations);
r = sm_ll_dec(&smm->ll, b, e, &nr_allocations);