slub: Dont define useless label in the !CONFIG_CMPXCHG_LOCAL case

The redo label needs #ifdeffery. Fixes the following problem introduced by
commit 8a5ec0ba42c4 ("Lockless (and preemptless) fastpaths for slub"):

  mm/slub.c: In function 'slab_free':
  mm/slub.c:2124: warning: label 'redo' defined but not used

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

mm/slub.c

@@ -2073,9 +2073,11 @@ static __always_inline void slab_free(struct kmem_cache *s,
 
 #ifndef CONFIG_CMPXCHG_LOCAL
 	local_irq_save(flags);
-#endif
 
+#else
 redo:
+#endif
+
 	/*
 	 * Determine the currently cpus per cpu slab.
 	 * The cpu may change afterward. However that does not matter since