rapidio: replace rio_free_net() with kfree() in rio_scan_alloc_net()

In the Linux kernel, the following vulnerability has been resolved:

rapidio: replace rio_free_net() with kfree() in rio_scan_alloc_net()

When idtab allocation fails, net is not registered with rio_add_net() yet,

so kfree(net) is sufficient to release the memory. Set mport->net to NULL

to avoid dangling pointer.