erofs: on-disk format should have explicitly assigned numbers

As Christoph suggested [1], on-disk format should have
explicitly assigned numbers.

[1] https://lore.kernel.org/r/20190829095954.GB20598@infradead.org/


Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-3-gaoxiang25@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>