x86/percpu: Force INVALID_PERCPU_AREA into the non-canonical address region

x86/percpu: Force INVALID_PERCPU_AREA into the non-canonical address region

This causes accidental uses of per_cpu() on a pcpu with an INVALID_PERCPU_AREA
to result in a #GF for attempting to access the middle of the non-canonical
virtual address region.

This is preferable to the current behaviour, where incorrect use of per_cpu()
will result in an effective NULL structure dereference which has security
implication in the context of PV guests.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>