slab: move up code to get kmem_cache_node in free_block()

node isn't changed, so we don't need to retreive this structure
everytime we move the object.  Maybe compiler do this optimization, but
making it explicitly is better.

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