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...

Reported-by: Yiming Qian <yimingqian591@gmail.com>
Fixes: 6b46ca260e22 ("net: psp: add socket security association code")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260427190606.366101-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>