io_uring: perform IOPOLL reaping if canceler is thread itself

We bypass IOPOLL completion polling (and reaping) for the SQPOLL thread,
but if it's the thread itself invoking cancelations, then we still need
to perform it or no one will.

Fixes: 9936c7c2

 ("io_uring: deduplicate core cancellations sequence")
Signed-off-by: Jens Axboe <axboe@kernel.dk>