io-wq: ensure all pending work is canceled on exit

io-wq: ensure all pending work is canceled on exit

If we race on shutting down the io-wq, then we should ensure that any
work that was queued after workers shutdown is canceled. Harden the
add work check a bit too, checking for IO_WQ_BIT_EXIT and cancel if
it's set.

Add a WARN_ON() for having any work before we kill the io-wq context.

Reported-by: [email protected]
Signed-off-by: Jens Axboe <[email protected]>