erofs: use dsb instead of layout for ondisk super_block

As Christoph pointed out [1], "Why is the variable name
for the on-disk subperblock layout? We usually still
calls this something with sb in the name, e.g. dsb.
for disksuper block. " Let's fix it.

[1] https://lore.kernel.org/r/20190829101545.GC20598@infradead.org/


Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-14-gaoxiang25@huawei.com


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