This commit adds the on-disk layout header file of erofs.

Note that the on-disk layout is still WIP, and some fields are
reserved for the future use by design.

Any comments are welcome.

Thanks-to: Li Guifu <liguifu2@huawei.com>
Thanks-to: Sun Qiuyang <sunqiuyang@huawei.com>
Signed-off-by: Miao Xie <miaoxie@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>