io_uring: fail cancellation for EXITING tasks

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

io_uring: fail cancellation for EXITING tasks

WARNING: CPU: 1 PID: 20 at fs/io_uring.c:6269 io_try_cancel_userdata+0x3c5/0x640 fs/io_uring.c:6269

CPU: 1 PID: 20 Comm: kworker/1:0 Not tainted 5.16.0-rc1-syzkaller #0

Workqueue: events io_fallback_req_func

RIP: 0010:io_try_cancel_userdata+0x3c5/0x640 fs/io_uring.c:6269

Call Trace:

<TASK>

io_req_task_link_timeout+0x6b/0x1e0 fs/io_uring.c:6886

io_fallback_req_func+0xf9/0x1ae fs/io_uring.c:1334

process_one_work+0x9b2/0x1690 kernel/workqueue.c:2298

worker_thread+0x658/0x11f0 kernel/workqueue.c:2445

kthread+0x405/0x4f0 kernel/kthread.c:327

ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

</TASK>

We need original task's context to do cancellations, so if it's dying

and the callback is executed in a fallback mode, fail the cancellation

attempt.