sched/numa: Fix boot crash on arm64 systems

Qian Cai reported a boot crash on arm64 systems, caused by:

  0fb3978 ("sched/numa: Fix NUMA topology for systems with CPU-less nodes")

The bug is that node_state() must be supplied a valid node_states[] array index,
but in task_numa_placement() the max_nid search can fail with NUMA_NO_NODE,
which is not a valid index.

Fix it by checking that max_nid is a valid index.

[ mingo: Added changelog. ]

Fixes: 0fb3978 ("sched/numa: Fix NUMA topology for systems with CPU-less nodes")
Reported-by: Qian Cai <[email protected]>
Tested-by: Qian Cai <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Linus Torvalds <[email protected]>
Signed-off-by: "Huang, Ying" <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

kernel/sched/fair.c

@@ -2437,7 +2437,7 @@ static void task_numa_placement(struct task_struct *p)
 	}
 
 	/* Cannot migrate task to CPU-less node */
-	if (!node_state(max_nid, N_CPU)) {
+	if (max_nid != NUMA_NO_NODE && !node_state(max_nid, N_CPU)) {
 		int near_nid = max_nid;
 		int distance, near_distance = INT_MAX;