defaults: Configure multi-gen LRU thrashing prevention

See https://www.kernel.org/doc/html/next/admin-guide/mm/multigen_lru.html

> Based on the average human detectable lag (~100ms), N=1000 usually eliminates
> intolerable janks due to thrashing. Larger values like N=3000 make janks less
> noticeable at the risk of premature OOM kills.