Fix up non-NUMA SLAB configuration for zero-sized allocations

I suspect Christoph tested his code only in the NUMA configuration, for
the combination of SLAB+non-NUMA the zero-sized kmalloc's would not work.

Of course, this would only trigger in configurations where those zero-
sized allocations happen (not very common), so that may explain why it
wasn't more widely noticed.

Seen by by Andi Kleen under qemu, and there seems to be a report by
Michael Tsirkin on it too.

Cc: Andi Kleen <[e-mail]>
Cc: Roland Dreier <[e-mail]>
Cc: Michael S. Tsirkin <[e-mail]>
Cc: Pekka Enberg <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/slab.c

···
 	 * functions.
 	 */
 	cachep = __find_general_cachep(size, flags);
-	if (unlikely(cachep == NULL))
-		return NULL;
+	if (unlikely(ZERO_OR_NULL_PTR(cachep)))
+		return cachep;
 	return __cache_alloc(cachep, flags, caller);
 }