SLUB: Make slub statistics use this_cpu_inc

this_cpu_inc() translates into a single instruction on x86 and does not
need any register. So use it in stat(). We also want to avoid the
calculation of the per cpu kmem_cache_cpu structure pointer. So pass
a kmem_cache pointer instead of a kmem_cache_cpu pointer.

Signed-off-by: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>