CVE-2026-98692: thunderbolt: Make the DP tunnel activation callback mandatory

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

thunderbolt: Make the DP tunnel activation callback mandatory

tbtunnelallocdp() takes an optional callback which is run from dprxwork once the DPRX capabilities read has completed. Without that callback tbdpdprxstart() reads the capabilities synchronously and never queues the work. It however always takes a tunnel reference which is only dropped by dprxwork itself or by tbdpdprxstop() when canceldelayedwork() actually canceled that work. That reference is thus leaked for every tunnel without a callback.

The only tunnels without one are those from tbtunneldiscoverdp(), which are activated again when restoring from hibernation. Pass the callback to tbtunneldiscoverdp() as well and drop the synchronous path such that the DPRX capabilities are always read from dprxwork. Hibernation restore then also no longer blocks for up to 12 seconds while waiting for that read to complete.

Also fix up the KUnit tests.