io_uring/io-wq: handle !sysctl_hung_task_timeout_secs

io_uring/io-wq: handle !sysctl_hung_task_timeout_secs

If the hung_task_timeout sysctl is set to 0, then we'll end up busy
looping inside io_wq_exit_workers() after an earlier commit switched to
using wait_for_completion_timeout(). Use the maximum schedule timeout
value for that case.

Fixes: 1f293098a313 ("io_uring/io-wq: don't trigger hung task for syzbot craziness")
Reported-by: Chris Mason <clm@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>