Revert "slub: do not put a slab to cpu partial list when cpu_partial is 0"

This reverts commit 318df36e57c0ca9f2146660d41ff28e8650af423.

This commit caused Steven Rostedt's hackbench runs to run out of memory
due to a leak.  As noted by Joonsoo Kim, it is buggy in the following
scenario:

 "I guess, you may set 0 to all kmem caches's cpu_partial via sysfs,
  doesn't it?

  In this case, memory leak is possible in following case.  Code flow of
  possible leak is follwing case.

   * in __slab_free()
   1. (!new.inuse || !prior) && !was_frozen
   2. !kmem_cache_debug && !prior
   3. new.frozen = 1
   4. after cmpxchg_double_slab, run the (!n) case with new.frozen=1
   5. with this patch, put_cpu_partial() doesn't do anything,
      because this cache's cpu_partial is 0
   6. return

  In step 5, leak occur"

And Steven does indeed have cpu_partial set to 0 due to RT testing.

Joonsoo is cooking up a patch, but everybody agrees that reverting this
for now is the right thing to do.

Reported-and-bisected-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>