KVM: x86/mmu: Drop non-PA bits when getting GFN for guest's PGD

KVM: x86/mmu: Drop non-PA bits when getting GFN for guest's PGD

Drop non-PA bits when getting GFN for guest's PGD with the maximum theoretical
mask for guest MAXPHYADDR.

Do it unconditionally because it's harmless for 32-bit guests, querying 64-bit
mode would be more expensive, and for EPT the mask isn't tied to guest mode.
Using PT_BASE_ADDR_MASK would be technically wrong (PAE paging has 64-bit
elements _except_ for CR3, which has only 32 valid bits), it wouldn't matter
in practice though.

Opportunistically use GENMASK_ULL() to define __PT_BASE_ADDR_MASK.

Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Tested-by: Xuelian Guo <xuelian.guo@intel.com>
Link: https://lore.kernel.org/r/20230913124227.12574-6-binbin.wu@linux.intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>