CVE-2025-22068: Use After Free

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

ublk: make sure ubq->canceling is set when queue is frozen

Now ublk driver depends on `ubq->canceling` for deciding if the request can be dispatched via uring_cmd & io_uring_cmd_complete_in_task().

Once ubq->canceling is set, the uring_cmd can be done via ublk_cancel_cmd() and io_uring_cmd_done().

So set ubq->canceling when queue is frozen, this way makes sure that the flag can be observed from ublk_queue_rq() reliably, and avoids use-after-free on uring_cmd.