blk-mq: defer to the normal submission path for post-flush requests

Requests with the FUA bit on hardware without FUA support need a post
flush before returning to the caller, but they can still be sent using
the normal I/O path after initializing the flush-related fields and
end I/O handler.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230519044050.107790-6-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -432,6 +432,17 @@ bool blk_insert_flush(struct request *rq)
 		 * Queue for normal execution.
 		 */
 		return false;
+	case REQ_FSEQ_DATA | REQ_FSEQ_POSTFLUSH:
+		/*
+		 * Initialize the flush fields and completion handler to trigger
+		 * the post flush, and then just pass the command on.
+		 */
+		blk_rq_init_flush(rq);
+		rq->flush.seq |= REQ_FSEQ_POSTFLUSH;
+		spin_lock_irq(&fq->mq_flush_lock);
+		list_move_tail(&rq->flush.list, &fq->flush_data_in_flight);
+		spin_unlock_irq(&fq->mq_flush_lock);
+		return false;
 	default:
 		/*
 		 * Mark the request as part of a flush sequence and submit it