io_uring: don't do extra EXITING cancellations

io_match_task() matches all requests with PF_EXITING task, even though
those may be valid requests. It was necessary for SQPOLL cancellation,
but now it kills all requests before exiting via
io_uring_cancel_sqpoll(), so it's not needed.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>