io_uring: fix handling SQEs requesting NOWAIT

Not all request types set REQ_F_FORCE_NONBLOCK when they needed async
punting; reverse logic instead and set REQ_F_NOWAIT if request mustn't
be punted.

Signed-off-by: Stefan Bühler <source@stbuehler.de>

Merged with my previous patch for this.

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