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/md.h b/drivers/md/md.h
index 2e0918f..b4f3352 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -424,7 +424,7 @@
 
 	/* Generic flush handling.
 	 * The last to finish preflush schedules a worker to submit
-	 * the rest of the request (without the REQ_FLUSH flag).
+	 * the rest of the request (without the REQ_PREFLUSH flag).
 	 */
 	struct bio *flush_bio;
 	atomic_t flush_pending;