mm, numa: Really disable NUMA balancing by default on single node machines

NUMA balancing is meant to be disabled by default on UMA machines but the check is using nr_node_ids (highest node) instead of num_online_nodes (online nodes). The consequences are that a UMA machine with a node ID of 1 or higher will enable NUMA balancing. This will incur useless overhead due to minor faults with the impact depending on the workload. These are the impact on the stats when running a kernel build on a single node machine whose node ID happened to be 1;

vanilla patched

NUMA base PTE updates          5113158           0
NUMA huge PMD updates              643           0
NUMA page range updates        5442374           0
NUMA hint faults               2109622           0
NUMA hint local faults         2109622           0

NUMA hint local percent 100 100
NUMA pages migrated 0 0

---
 mm/mempolicy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index ede26291d4aa..747743237d9f 100644
@@ -2518,7 +2518,7 @@ static void __init check_numabalancing_enable(void)
 	if (numabalancing_override)
 		set_numabalancing_state(numabalancing_override == 1);
 
-	if (nr_node_ids > 1 && !numabalancing_override) {
+	if (num_online_nodes() > 1 && !numabalancing_override) {
 		pr_info("%s automatic NUMA balancing. "
 			"Configure with numa_balancing= or the "
 			"kernel.numa_balancing sysctl",

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