KVM: x86/mmu: Derive page role for TDP MMU shadow pages from parent

Derive the page role from the parent shadow page, since the only thing
that changes is the level. This is in preparation for splitting huge
pages during VM-ioctls which do not have access to the vCPU MMU context.

No functional change intended.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220119230739.2234394-14-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>