KVM: x86/mmu: Use common logic for computing the 32/64-bit base PA mask

Use common logic for computing PT_BASE_ADDR_MASK for 32-bit, 64-bit, and
EPT paging.  Both PAGE_MASK and the new-common logic are supsersets of
what is actually needed for 32-bit paging.  PAGE_MASK sets bits 63:12 and
the former GUEST_PT64_BASE_ADDR_MASK sets bits 51:12, so regardless of
which value is used, the result will always be bits 31:12.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220614233328.3896033-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>