slab: fix for_each_kmem_cache_node()

Fix a bug (discovered with kmemcheck) in for_each_kmem_cache_node().  The
for loop reads the array "node" before verifying that the index is within
the range.  This results in kmemcheck warning.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>