lc_element_by_index
e = lc_element_by_index(device->resync, i);
extent_nr = lc_element_by_index(device->act_log, idx)->lc_number;
al_ext = lc_element_by_index(device->act_log, i);
e = lc_element_by_index(t, i);
extern struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i);
e = lc_element_by_index(lc, i);
EXPORT_SYMBOL(lc_element_by_index);