test/io-cancel: -ECANCELED is a valid return value

-EINTR relies on io-wq, we can also find and cancel before that. Make
sure the test allows it, fixing the following case failure on newer
kernels:

1 -125
child failed 1
test_cancel_req_across_fork() failed

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