io-wq: check for wq exit after adding new worker task_work

We check IO_WQ_BIT_EXIT before attempting to create a new worker, and
wq exit cancels pending work if we have any. But it's possible to have
a race between the two, where creation checks exit finding it not set,
but we're in the process of exiting. The exit side will cancel pending
creation task_work, but there's a gap where we add task_work after we've
canceled existing creations at exit time.

Fix this by checking the EXIT bit post adding the creation task_work.
If it's set, run the same cancelation that exit does.

Reported-and-tested-by:  <syzbot+b60c982cb0efc5e05a47@syzkaller.appspotmail.com>
Reviewed-by: Hao Xu <haoxu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>