From: Manfred Spraul <manfred@colorfullife.com>

The slab allocator keeps track of the free objects in a slab with a linked
list of integers (typedef'ed to kmem_bufctl_t).  Right now unsigned int is
used for kmem_bufctl_t, i.e.  4 bytes per-object overhead.

The attached patch implements a per-arch definition of for this type:
Theoretically, unsigned short is sufficient for kmem_bufctl_t and this would
reduce the per-object overhead to 2 bytes.  But some archs cannot operate on
16-bit values efficiently, thus it's not possible to switch everyone to
ushort.

The chosen types are a result of dicussions with the various arch maintainers.