Patch from Manfred Spraul.

Fixes a bug which was exposed by Zwane's hotplug CPU work.  The
cache_cache.array pointer is initially given a temp bootstrap area, which is
later converted over to the final value after the CPU is brought up.

But if slab is enhanced to permit cancellation of a CPU bringup, this pointer
ends up pointing at stale memory.  So reinitialise it by hand when
kmem_cache_init() is run.