powerpc/numa: Check for all VPHN changes

The hypervisor uses unsigned 1 byte counters to signal topology changes to
the OS. Since they can wrap we need to check for any difference, not just if
the hypervisor count is greater than the previous count.

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