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>
diff --git a/drivers/md/dm-era-target.c b/drivers/md/dm-era-target.c
index 665bf32..2faf49d8 100644
--- a/drivers/md/dm-era-target.c
+++ b/drivers/md/dm-era-target.c
@@ -1540,9 +1540,9 @@
 	remap_to_origin(era, bio);
 
 	/*
-	 * REQ_FLUSH bios carry no data, so we're not interested in them.
+	 * REQ_PREFLUSH bios carry no data, so we're not interested in them.
 	 */
-	if (!(bio->bi_rw & REQ_FLUSH) &&
+	if (!(bio->bi_rw & REQ_PREFLUSH) &&
 	    (bio_data_dir(bio) == WRITE) &&
 	    !metadata_current_marked(era->md, block)) {
 		defer_bio(era, bio);