io_uring/io-wq: add exit-on-idle state

io_uring/io-wq: add exit-on-idle state

io-wq uses an idle timeout to shrink the pool, but keeps the last worker
around indefinitely to avoid churn.

For tasks that used io_uring for file I/O and then stop using io_uring,
this can leave an iou-wrk-* thread behind even after all io_uring
instances are gone. This is unnecessary overhead and also gets in the
way of process checkpoint/restore.

Add an exit-on-idle state that makes all io-wq workers exit as soon as
they become idle, and provide io_wq_set_exit_on_idle() to toggle it.

Signed-off-by: Li Chen <me@linux.beauty>
Signed-off-by: Jens Axboe <axboe@kernel.dk>