cake_heap_get_backlog
u32 mb = cake_heap_get_backlog(q, i);
u32 lb = cake_heap_get_backlog(q, l);
u32 rb = cake_heap_get_backlog(q, r);
u32 ib = cake_heap_get_backlog(q, i);
u32 pb = cake_heap_get_backlog(q, p);