md: don't reference gendisk in getgeo

Using ->array_sectors rather than get_capacity() is more
direct and is a step towards relaxing the tight connection
between mddev and gendisk.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -5556,7 +5556,7 @@ static int md_getgeo(struct block_device *bdev, struct hd_geometry *geo)
 
 	geo->heads = 2;
 	geo->sectors = 4;
-	geo->cylinders = get_capacity(mddev->gendisk) / 8;
+	geo->cylinders = mddev->array_sectors / 8;
 	return 0;
 }