swim3 has at most single request in flight and already tracks it using
fd_req.  Convert it to dequeuing model by updating request fetching
and wrapping completion function.

[ Impact: dequeue in-flight request ]
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>