ublk: fix race between io_uring_cmd_complete_in_task and ublk_cancel_cmd

ublk: fix race between io_uring_cmd_complete_in_task and ublk_cancel_cmd

ublk_cancel_cmd() calls io_uring_cmd_done() to complete uring_cmd, but
we may have scheduled task work via io_uring_cmd_complete_in_task() for
dispatching request, then kernel crash can be triggered.

Fix it by not trying to canceling the command if ublk block request is
started.

Fixes: 216c8f5ef0f2 ("ublk: replace monitor with cancelable uring_cmd")
Reported-by: Jared Holzman <jholzman@nvidia.com>
Tested-by: Jared Holzman <jholzman@nvidia.com>
Closes: https://lore.kernel.org/linux-block/d2179120-171b-47ba-b664-23242981ef19@nvidia.com/
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250425013742.1079549-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>