exception_hash
static uint32_t exception_hash(struct dm_exception_table *et, chunk_t chunk);
lock->complete_slot = &complete->table[exception_hash(complete, chunk)].lock;
lock->pending_slot = &pending->table[exception_hash(pending, chunk)].lock;
slot = &et->table[exception_hash(et, chunk)].head;
l = &eh->table[exception_hash(eh, new_e->old_chunk)].head;