io_uring: kill io_poll_issue's PF_EXITING check

We don't need to worry about checking PF_EXITING in io_poll_issue().
task works using the function should take care of it and never try to
resubmit / retry if the task is dying.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2e9dc998dc07507c759a0c9cb5d2fbea0710d58c.1669821213.git.asml.silence@gmail.com


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