io_uring: fail poll arm on queue proc failure

Check the ipt.error value, it must have been either cleared to zero or
set to another error than the default -EINVAL if we don't go through the
waitqueue proc addition. Just give up on poll at that point and return
failure, this will fallback to async work.

io_poll_add() doesn't suffer from this failure case, as it returns the
error value directly.

Cc: stable@vger.kernel.org # v5.7+
Reported-by:  <syzbot+a730016dc0bdce4f6ff5@syzkaller.appspotmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>