Abstract calculating the size from calculating the available size
Abstract calculating the size from calculating the available size

The value is useful on its own.

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

stratis-storage/stratisd,trgill/stratisd,mulkieran/stratisd,trgill/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,mulkieran/stratisd