Add some more constants and a test
Add some more constants and a test

The constants represent sizes of some parts of the MDA regions, the
tests verify that they have the correct relationship to each other.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd