Local kernel warning via incorrect IDR emptiness check

Incorrect IDR state handling in Linux 6.12 through 6.12.1 allows local users to trigger a kernel warning during L2TP namespace teardown. The l2tp_exit_net handler calls idr_is_empty on an IDR whose radix tree can retain only internal nodes after idr_alloc_32 allocation failure, so the check falsely reports live entries before idr_destroy removes those nodes. Reaching the warning requires low-privileged local access, induced allocation failure, and L2TP network-namespace teardown; service interruption depends on a configuration that panics on kernel warnings.