cbfs: do not pack struct cbfs_cachenode

With the __packed attribute sandbox_defconfig cannot be compiled with GCC
9.2.1:

fs/cbfs/cbfs.c: In function ‘file_cbfs_fill_cache’:
fs/cbfs/cbfs.c:164:16: error: taking address of packed member of
‘struct cbfs_cachenode’ may result in an unaligned pointer value
[-Werror=address-of-packed-member]
  164 |   cache_tail = &new_node->next;
      |                ^~~~~~~~~~~~~~~

struct cbfs_cachenode is only an internal structure. So let's rearrange the
fields such that the structure is naturally packed and remove the __packed
attribute.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>