io_uring: make ctx cancel on exit targeted to actual ctx

Before IORING_SETUP_ATTACH_WQ, we could just cancel everything on the
io-wq when exiting. But that's not the case if they are shared, so
cancel for the specific ctx instead.

Cc: stable@vger.kernel.org
Fixes: 24369c2e ("io_uring: add io-wq workqueue sharing")
Signed-off-by: Jens Axboe <axboe@kernel.dk>