ataflop: dequeue and track in-flight request

ataflop has single request in flight.  Till now, whenever it needs to
access the in-flight request it called elv_next_request().  This patch
makes ataflop track the in-flight request directly and dequeue it when
processing starts.  The added complexity is minimal and this will help
future block layer changes.

[ Impact: dequeue in-flight request, one elv_next_request() per request ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>