io_uring: don't use complete_all() on SQPOLL thread exit

We want to reuse this completion, and a single complete should do just
fine. Ensure that we park ourselves first if requested, as that is what
lead to the initial deadlock in this area. If we've got someone attempting
to park us, then we can't proceed without having them finish first.

Fixes: 37d1e2e3

 ("io_uring: move SQPOLL thread io-wq forked worker")
Signed-off-by: Jens Axboe <axboe@kernel.dk>