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().

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>