numa: fix NULL pointer access and memory leak in unregister_one_node()

When doing socket hot remove, "node_devices[nid]" is set to NULL; acpi_processor_remove()
try_offline_node()
unregister_one_node()

Then hot add a socket, but do not echo 1 > /sys/devices/system/cpu/cpuXX/online, so register_one_node() will not be called, and "node_devices[nid]" is still NULL.

If doing socket hot remove again, NULL pointer access will be happen. unregister_one_node()
unregister_node()

Another, we should free the memory used by "node_devices[nid]" in unregister_one_node().

---
 drivers/base/node.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

index bc9f43b..8f7ed99 100644
@@ -599,7 +599,11 @@ int register_one_node(int nid)
 
 void unregister_one_node(int nid)
 {
+	if (!node_devices[nid])
+		return;
+
 	unregister_node(node_devices[nid]);
+	kfree(node_devices[nid]);
 	node_devices[nid] = NULL;
 }
 

1.7.1