erofs: refine on-disk definition comments

Fix some outdated comments and typos, hopefully helpful.

Link: https://lore.kernel.org/r/20220506194612.117120-3-hsiangkao@linux.alibaba.com


Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>