slub: fix per cpu kmem_cache_cpu array memory leak

The per cpu array of kmem_cache_cpu structures accomodates
NR_KMEM_CACHE_CPU such structs.

When this array overflows and a struct is allocated by kmalloc(), it may
have an address at the upper bound of this array.  If this happens, it
does not get freed and the per cpu kmem_cache_cpu_free pointer will be out
of bounds after kmem_cache_destroy() or cpu offlining.

Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>