need_gc
ca->set->need_gc = max(ca->set->need_gc, bucket_gc_gen(b));
WARN_ON_ONCE(ca->set->need_gc > BUCKET_GC_GEN_MAX);
uint8_t need_gc;
c->need_gc = 0;
c->need_gc = max(c->need_gc, bucket_gc_gen(b));
stale, b->c->need_gc);
if (!need_gc(cache, &dirty_tail, &key_tail))