x86/spec-ctrl: Calculate safe PTE addresses for L1TF mitigations

x86/spec-ctrl: Calculate safe PTE addresses for L1TF mitigations

Safe PTE addresses for L1TF mitigations are ones which are within the L1D
address width (may be wider than reported in CPUID), and above the highest
cacheable RAM/NVDIMM/BAR/etc.

All logic here is best-effort heuristics, which should in practice be fine for
most hardware.  Future work will see about disentangling the SRAT handling
further, as well as having L0 pass this information down to lower levels when
virtualised.

This is part of XSA-273 / CVE-2018-3620.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>