Linux: thunderbolt: Hold a router reference for each allocated HopID

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

thunderbolt: Hold a router reference for each allocated HopID

tb_stop() drops the reference to all DP tunnels but does not deactivate them, thus nothing cancels a dprx_work still in flight (which holds its own tunnel reference) and the tunnel can outlive tb_switch_remove(). The HopID releases in tb_path_free() then operate on freed IDAs and trigger warnings like

ida_free called for id=8 which is not allocated.

This can be triggered by unbinding the driver while a DP tunnel is still waiting for the DPRX capabilities read to finish. On the Apple NHI unplugging the cable runs into just that reliably because the read can never finish right now and because the unplug powers down the entire USB4 complex and removes the NHI device.

Take a router reference whenever an input or output HopID is allocated and drop it again after the HopID is released. This keeps the ports and their HopID IDAs alive for as long as they are used.

The KUnit tests allocate routers without ever registering their devices so initialize the embedded struct device there as well and drop its initial reference when the test is finished.