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:  <syzbot+91b4b56ead187d35c9d3@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>