…and dir_index

Inode 257 is only with inode_extref without inode_ref.
And This case contains an inode_extref:
==========================================
...
   item 1 key (257 INODE_EXTREF 3460996356) itemoff 3947 itemsize 24
                index 257 parent 256 namelen 6 name: foo255
...
==========================================
The related dir_item and dir_index are missing.

Add the case to ensure both original and lowmem mode check can handle
the case of inode_extref.

Lowmem part is supported since patch named
'btrfs-progs: lowmem: optimization and repair for check_inode_extref()'.

Rename default_case.img to inode_ref_without_dir_item_and_index.img.

Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>