io_uring/cancel: don't spin on local work the task can't run

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

io_uring/cancel: don't spin on local work the task can't run

io_uring_cancel_generic() refuses to sleep while any ring on the exiting
task's tctx has local work pending. It assumes the task is that ring's
submitter and that the next io_uring_try_cancel_requests() pass will run
the work. This does not hold for a ring that was still
IORING_SETUP_R_DISABLED when the task got its tctx node from
io_uring_create() or io_ringfd_register(). The submitter is set later by
IORING_REGISTER_ENABLE_RINGS, possibly from another task, and only the
submitter may run local work on an IORING_SETUP_DEFER_TASKRUN ring. To
the exiting task the work stays pending forever, the loop never reaches
schedule(), and the task trips the WARN_ON_ONCE() once and then spins in
do_exit().

An unprivileged user can set this up at will by creating a disabled
ring, having a child enable it and stop, posting to the ring with
IORING_OP_MSG_RING and exiting with a request in flight that only the
child can complete. The exiting thread burns a CPU until the child runs
or dies, and with panic_on_warn set the WARN panics the kernel.

Fix by skipping the sleep only when io_allowed_defer_tw_run() allows the
task to run the work. This is reasonable since work it cannot run cannot
complete any of its requests either, so nothing is lost by sleeping.
Drop the WARN_ON_ONCE() since handing a disabled ring to another thread
is a supported scenario, not a bug state.