io-wq: ensure work->task_pid is cleared on init

We use ->task_pid for exit cancellation, but we need to ensure it's
cleared to zero for io_req_work_grab_env() to do the right thing. Take
a suggestion from Bart and clear the whole thing, just setting the
function passed in. This makes it more future proof as well.

Fixes: 36282881 ("io-wq: add io_wq_cancel_pid() to cancel based on a specific pid")
Signed-off-by: Jens Axboe <axboe@kernel.dk>