io_uring: clean poll tw PF_EXITING handling

When we meet PF_EXITING in io_poll_check_events(), don't overcomplicate
the code with io_poll_mark_cancelled() but just return -ECANCELED and
the callers will deal with the rest.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f0cc981af82a5b193658f8f44397eeb3bf838b7b.1650056133.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>