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 order-0
allocations to reduce unnecessary overhead.