ACPI: NUMA: Fix the logic of getting the fake_pxm value

The for loop does not iterate over the last element of the node_to_pxm_map
array. This could lead to a conflict between the final fake_pxm value and
the existing pxm values. That is, the final fake_pxm value can not be
guaranteed to be an unused pxm value.

While at it, fix up white space in slit_valid().

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>