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

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.