cpumask: replace node_to_cpumask with cpumask_of_node.

Impact: cleanup

node_to_cpumask (and the blecherous node_to_cpumask_ptr which
contained a declaration) are replaced now everyone implements
cpumask_of_node.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

mm/slab.c

@@ -1160,7 +1160,7 @@ static void __cpuinit cpuup_canceled(long cpu)
 	struct kmem_cache *cachep;
 	struct kmem_list3 *l3 = NULL;
 	int node = cpu_to_node(cpu);
-	node_to_cpumask_ptr(mask, node);
+	const struct cpumask *mask = cpumask_of_node(node);
 
 	list_for_each_entry(cachep, &cache_chain, next) {
 		struct array_cache *nc;