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

psp_assoc_device_get_locked() obtains a psp_dev reference via

psp_dev_get_for_sock() (which uses psp_dev_tryget() under RCU);

it then acquires psd->lock and drops the reference. Before

the lock is taken, psp_dev_unregister() 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. psp_dev_is_registered() exists

already but had no callers, which makes me wonder if I either

forgot to add this or lost the check during refactoring...