ublk: fix race between io_uring_cmd_complete_in_task and ublk_cancel_cmd

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

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.