SLUB: Fix per-cpu merge conflict

The slab tree adds a percpu variable usage case (commit
9dfc6e68 "SLUB: Use this_cpu operations in
slub"), but the percpu tree removes the prefixing of percpu variables (commit
dd17c8f7

 "percpu: remove per_cpu__ prefix"),
thus causing the following compilation error:

    CC      mm/slub.o
  mm/slub.c: In function ‘alloc_kmem_cache_cpus’:
  mm/slub.c:2078: error: implicit declaration of function ‘per_cpu_var’
  mm/slub.c:2078: warning: assignment makes pointer from integer without a cast
  make[1]: *** [mm/slub.o] Error 1

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>