powerpc/kvm: Fix kvm_use_magic_page

When switching from __get_user to fault_in_pages_readable, commit
9f9eae5c broke kvm_use_magic_page: like __get_user,
fault_in_pages_readable returns 0 on success.

Fixes: 9f9eae5c ("powerpc/kvm: Prefer fault_in_pages_readable function")
Cc: stable@vger.kernel.org # v4.18+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>