slab: use get_node() and kmem_cache_node() functions
Use the two functions to simplify the code avoiding numerous explicit
checks coded checking for a certain node to be online.
Get rid of various repeated calculations of kmem_cache_node structures.
Signed-off-by: Christoph Lameter <cl@linux.com> Cc: Christoph Lameter <cl@linux.com> Cc: Pekka Enberg <penberg@kernel.org> Acked-by: David Rientjes <rientjes@google.com> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>