erofs: update erofs_fs.h comments

As Christoph said [1] [2], update it now.

[1] https://lore.kernel.org/r/20190902124521.GA22153@infradead.org/
[2] https://lore.kernel.org/r/20190902120548.GB15931@infradead.org/


Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-11-gaoxiang25@huawei.com


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