KVM: guest_memfd: Call arch make_shared callback for to-shared conversion

private 转 shared 时遍历受影响 folio，调用 kvm_arch_gmem_make_shared() 清理平台私有状态/加密元数据；在 MMU 失效开始后调用以阻止并发页故障；不实现 make-private，因 SNP 偏好 fault 时再 private。