rbd: use the correct length for format 2 object names

Format 2 objects use 16 characters for the object name suffix to be
able to express the full 64-bit range of object numbers. Format 1
images only use 12 characters for this. Using 12-character names for
format 2 caused userspace and kernel rbd clients to read differently
named objects, which made an image written by one client look empty to
the other client.

CC: [e-mail] # 3.9+
Reported-by: Chris Dunlop <[e-mail]>
Signed-off-by: Josh Durgin <[e-mail]>
Reviewed-by: Sage Weil <[e-mail]>

drivers/block/rbd.c

···
 	char *name;
 	u64 segment;
 	int ret;
+	char *name_format;
 
 	name = kmem_cache_alloc(rbd_segment_name_cache, GFP_NOIO);
 	if (!name)
 		return NULL;
 	segment = offset >> rbd_dev->header.obj_order;
-	ret = snprintf(name, MAX_OBJ_NAME_SIZE + 1, "%s.%012llx",
+	name_format = "%s.%012llx";
+	if (rbd_dev->image_format == 2)
+		name_format = "%s.%016llx";
+	ret = snprintf(name, MAX_OBJ_NAME_SIZE + 1, name_format,
 			rbd_dev->header.object_prefix, segment);
 	if (ret < 0 || ret > MAX_OBJ_NAME_SIZE) {
 		pr_err("error formatting segment name for #%llu (%d)\n",