swim processes requests one-by-one synchronously and can easily be
converted to dequeuing model.  Convert it.

[ Impact: dequeue in-flight request ]
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Laurent Vivier <Laurent@lvivier.info>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>