slub: set slub_max_order to 0

    To avoid locking and per-cpu overhead, SLUB optimisically uses
    high-order allocations up to order-3 by default and falls back to lower
    allocations if they fail.

    So, with the default value (slub_max_order = 3), the kernel may ask up
    to 3 contigous pages for each slab. However, with a reduced amount of
    memory it is not worth to try to do order-3 allocations, it adds more
    overhead (shrinkers have to free more objects to release any memory) and
    probably it can easily fail to success with order-3 allocations.

    For this reason it is better to immediately fallback to oder-0
    allocations to reduce unnecessary overhead.