rbd: only set disk to read-only once

rbd_open(), called every time the device is opened, calls
set_device_ro().  There's no reason to set the device read-only or
read-write every time it is opened. Just do this once during device
setup, using set_disk_ro() instead because the struct block_device
isn't available to us there.
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>

drivers/block/rbd.c

@@ -541,7 +541,6 @@ static int rbd_open(struct block_device *bdev, fmode_t mode)
 		return -ENOENT;
 
 	(void) get_device(&rbd_dev->dev);
-	set_device_ro(bdev, rbd_dev->mapping.read_only);
 
 	return 0;
 }
@@ -5060,6 +5059,7 @@ static int rbd_dev_device_setup(struct rbd_device *rbd_dev)
 	if (ret)
 		goto err_out_disk;
 	set_capacity(rbd_dev->disk, rbd_dev->mapping.size / SECTOR_SIZE);
+	set_disk_ro(rbd_dev->disk, rbd_dev->mapping.read_only);
 
 	ret = rbd_bus_add_dev(rbd_dev);
 	if (ret)