Per cpu structures for SLUB

The following patchset introduces per cpu structures for SLUB. These
are very small (and multiples of these may fit into one cacheline)
and (apart from performance improvements) allow the addressing of
several isues in SLUB:

1. The number of objects per slab is no longer limited to a 16 bit
   number.

2. Room is freed up in the page struct. We can avoid using the
   mapping field which allows to get rid of the #ifdef CONFIG_SLUB
   in page_mapping().

3. We will have an easier time adding things like Peter Z.s reserve
   management (hopefully not).

Performance
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