CVE-2025-21934 rapidio: fix an API misues when rio_add_net() fails

In the Linux kernel, the following vulnerability has been resolved: rapidio: fix an API misues when rioaddnet fails rioaddnet calls deviceregister and fails when deviceregister fails. Thus, putdevice should be used rather than kfree. Add "mport-net = NULL;" to avoid a use after free issue...