erofs: move packed inode out of the compression part

packed inode could be used in more scenarios which are independent of
compression in the future.

For example, packed inode could be used to keep extra long xattr
prefixes with the help of following patches.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230407141710.113882-4-jefflexu@linux.alibaba.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>