erofs: add __packed annotation to union(__le16..)

I'm unsure why they aren't 2 bytes in size only in arm-linux-gnueabi.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202504051202.DS7QIknJ-lkp@intel.com


Fixes: 61ba89b5 ("erofs: add 48-bit block addressing on-disk support")
Fixes: efb2aef5 ("erofs: add encoded extent on-disk definition")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250408114448.4040220-1-hsiangkao@linux.alibaba.com