io_uring/wait: don't spin on a stuck overflow list

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

io_uring/wait: don't spin on a stuck overflow list

As long as IO_CHECK_CQ_OVERFLOW_BIT is set, io_cqring_wait_schedule()
returns right away, before it gets to look at signals or arms the timeout,
for io_cqring_wait() to flush the overflow list. If that doesn't empty the
list, and the CQ ring doesn't hold the number of events the caller wants,
the loop starts over, with the same result. The task spins in the kernel,
and can't be killed.

The assumption is that this can't happen: if CQEs can't be flushed, the
ring is full, and min_events is capped to the size of the ring. But:

- With IORING_SETUP_CQE_MIXED a 32b CQE needs two contiguous entries. It
is stuck on the list if a single one is free, and then the ring holds
less than cq_entries events. A single threaded application that asks
for as many events as the ring has entries never gets out of
io_uring_enter().

- Whether the ring is full is up to the CQ head, which the application
can write to. While one thread waits for events on an empty ring,
another one can move the head back so that the ring looks full and
have a CQE overflow. The waiter can't be stopped any more, not even by
SIGKILL.

Only the application can make room in the CQ ring. Return to it if the
list could not be flushed.