rbd: rename block_name -> object_prefix

In the on-disk image header structure there is a field "block_name"
which represents what we now call the "object prefix" for an rbd
image.  Rename this field "object_prefix" to be consistent with
modern usage.

This appears to be the only remaining vestige of the use of "block"
in symbols that represent objects in the rbd code.

This addresses http://tracker.newdream.net/issues/1761

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>

drivers/block/rbd.c

@@ -522,11 +522,11 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 
 	snap_count = le32_to_cpu(ondisk->snap_count);
 
-	size = sizeof (ondisk->block_name) + 1;
+	size = sizeof (ondisk->object_prefix) + 1;
 	header->object_prefix = kmalloc(size, GFP_KERNEL);
 	if (!header->object_prefix)
 		return -ENOMEM;
-	memcpy(header->object_prefix, ondisk->block_name, size - 1);
+	memcpy(header->object_prefix, ondisk->object_prefix, size - 1);
 	header->object_prefix[size - 1] = '\0';
 
 	if (snap_count) {

drivers/block/rbd_types.h

@@ -47,7 +47,7 @@ struct rbd_image_snap_ondisk {
 
 struct rbd_image_header_ondisk {
 	char text[40];
-	char block_name[24];
+	char object_prefix[24];
 	char signature[4];
 	char version[8];
 	struct {