io_uring/waitid: prevent cancel reference wrap

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

io_uring/waitid: prevent cancel reference wrap

io_waitid packs the cancellation state into bit 31 of ->refs and uses
bits 30:0 as a 31-bit reference counter:

#define IO_WAITID_CANCEL_FLAG BIT(31)
#define IO_WAITID_REF_MASK GENMASK(30, 0)

__io_waitid_cancel() marks a request canceled, then increments the low
bits of ->refs to claim completion ownership. When another path owns the
request, the increment remains even though cancellation returns false.
The request remains discoverable by cancellation, so repeated failed
attempts can wrap the 31-bit reference field to zero. A later
cancellation can claim ownership and complete the request a second time.

Use atomic_fetch_or() when setting the cancel flag and inspect the old
flag value. The first cancel attempt continues to the existing ownership
increment, preserving its wake and issuer handoff semantics. Later
attempts see the cancel flag and return without incrementing the
reference field.

This makes repeated failed cancellation attempts idempotent while
leaving the first-cancel state transitions unchanged.