io_uring: move SQPOLL thread io-wq forked worker

      Don't use a kthread for SQPOLL, use a forked worker just like the io-wq
workers. With that done, we can drop the various context grabbing we do
for SQPOLL, it already has everything it needs.

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