io_uring: ensure we can nest attempts to park SQPOLl thread

io_uring: ensure we can nest attempts to park SQPOLl thread

Since we can now share it, we need to be careful that we nest parks
properly. Otherwise we can hit spew from kthread on attempting to park
and already parked thread.

Protect parking with a mutex, so new parkers get to wait in line.

Signed-off-by: Jens Axboe <axboe@kernel.dk>