CVE-2026-52979: net: psp: check for device unregister when creating assoc

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

net: psp: check for device unregister when creating assoc

pspassocdevicegetlocked() obtains a pspdev reference via pspdevgetforsock() (which uses pspdevtryget() under RCU); it then acquires psd->lock and drops the reference. Before the lock is taken, pspdevunregister() can run to completion: take psd->lock, clear out state, unlock, drop the registration reference.

The expectation is that the lock prevents device unregistration, but much like with netdevs special care has to be taken when "upgrading" a reference to a locked device. Add the missing check if device is still alive. pspdevisregistered() exists already but had no callers, which makes me wonder if I either forgot to add this or lost the check during refactoring...