A slab allocator (removal) update

There's been numerous attempts to improve the situation over the years, LWN articles about it even, but it never makes progress since it's a relatively niche situation without enough visibility to most kernel devs to maintain momentum to improve the situation. I'm not even sure there's any mechanism to stop the dentry cache purge when memory-pressure triggers it... it may literally scan ALL of cache and purge what it can instead of freeing say a few MB and stopping, I haven't checked the code in depth but that's the general behavior I've seen in the past.