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-io.c b/drivers/md/dm-io.c
index 22e0597..0e225fd 100644
--- a/drivers/md/dm-io.c
+++ b/drivers/md/dm-io.c
@@ -380,7 +380,7 @@
 	 */
 	for (i = 0; i < num_regions; i++) {
 		*dp = old_pages;
-		if (where[i].count || (op_flags & REQ_FLUSH))
+		if (where[i].count || (op_flags & REQ_PREFLUSH))
 			do_region(op, op_flags, i, where + i, dp, io);
 	}