powerpc/numa: Fix bug in unmap_cpu_from_node

When converting to the new cpumask code I screwed up:

-       if (cpu_isset(cpu, numa_cpumask_lookup_table[node])) {
-               cpu_clear(cpu, numa_cpumask_lookup_table[node]);
+       if (cpumask_test_cpu(cpu, node_to_cpumask_map[node])) {
+               cpumask_set_cpu(cpu, node_to_cpumask_map[node]);

This was introduced in commit 25863de0 (powerpc/cpumask: Convert NUMA code
to new cpumask API)

Fix it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: <stable@kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>