block, drivers, fs: rename REQ_FLUSH to REQ_PREFLUSH

To avoid confusion between REQ_OP_FLUSH, which is handled by
request_fn drivers, and upper layers requesting the block layer
perform a flush sequence along with possibly a WRITE, this patch
renames REQ_FLUSH to REQ_PREFLUSH.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/md/raid10.c

@@ -1447,7 +1447,7 @@ static void raid10_make_request(struct mddev *mddev, struct bio *bio)
 
 	struct bio *split;
 
-	if (unlikely(bio->bi_rw & REQ_FLUSH)) {
+	if (unlikely(bio->bi_rw & REQ_PREFLUSH)) {
 		md_flush_request(mddev, bio);
 		return;
 	}