mg_disk has at most single request in flight per device. Till now,
whenever it needs to access the in-flight request it called
elv_next_request(). This patch makes mg_disk track the in-flight
request directly using mg_host->req and dequeue it when processing
starts.
q->queuedata is set to mg_host so that mg_host can be determined
without fetching request from the queue.
[ Impact: dequeue in-flight request, one elv_next_request() per request ]