[$] Removing a pointer dereference from slab allocations

Al Viro does not often stray outside of the core virtual filesystem area;when he does, it is usually worthy of note. Recently, he wandered intomemory management with this patchseries to the slab allocator and some of its users. Kernel developerswill often put considerable effort into small optimizations, but it isstill interesting to look at just how much effort has gone toward the purpose ofavoiding a single pointer dereference in some memory-allocation hot paths.