ACPI: Remove side effect of partly creating a node in acpi_map_pxm_to_online_node()

While this function will only return an online node, it can have the side
effect of partially creating a new node.  The existing comments suggest this
is intentional, but the usecases of this function are related to NFIT and
HMAT parsing, neither of which should be able to define new nodes.

One route by which the existing behaviour would cause a crash is to have a
_PXM entry in ACPI DSDT attempt to place a device within this partly
created proximity domain. A subsequent call to devm_kzalloc() or similar
would result in an attempt to allocate memory on a node for which zone
lists have not been set up and a NULL pointer dereference.

Prevent such cases by switching to pxm_to_node() within
acpi_map_pxm_to_online_node() which cannot cause a new node to be
partly created. If one would previously have been created we now
return NO_NUMA_NODE.  Documentation updated to reflect this change.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>