Denial of service via unreset rqspinlock queue tail

Denial of service in Linux kernel 6.19 allows local users to cause an indefinite rqspinlock stall during ABBA deadlock handling. The deadlock path in kernel/bpf/rqspinlock.c preserves the waiter queue without resetting its tail, so the final waiter waits for a node->next link that may never arrive. A low-privileged local user must trigger the specific ABBA locking sequence; the resulting delay is intermittent when another waiter joins and indefinite otherwise.