block: flush: fix IO hang in case of flood fua req

	struct request_queue *q = rq->q;

	struct blk_flush_queue *fq = blk_get_flush_queue(q, NULL);

	/*

	 * Updating q->in_flight[] here for making this tag usable

	 * early. Because in blk_queue_start_tag(),

	 * q->in_flight[BLK_RW_ASYNC] is used to limit async I/O and

	 * reserve tags for sync I/O.

	 *

	 * More importantly this way can avoid the following I/O

	 * deadlock:

	 *

	 * - suppose there are 40 fua requests comming to flush queue

	 *   and queue depth is 31

	 * - 30 rqs are scheduled then blk_queue_start_tag() can't alloc

	 *   tag for async I/O any more

	 * - all the 30 rqs are completed before FLUSH_PENDING_TIMEOUT

	 *   and flush_data_end_io() is called

	 * - the other rqs still can't go ahead if not updating

	 *   q->in_flight[BLK_RW_ASYNC] here, meantime these rqs

	 *   are held in flush data queue and make no progress of

	 *   handling post flush rq

	 * - only after the post flush rq is handled, all these rqs

	 *   can be completed

	 */

	elv_completed_request(q, rq);

	/* for avoiding double accounting */

	rq->cmd_flags &= ~REQ_STARTED;

	/*

	 * After populating an empty queue, kick it to avoid stall.  Read

	 * the comment in flush_end_io().