#86 File system data structure checksums

[kfairbanks]

Ext4 stores checksums for various structures, I will assume other file systems do the same. It might be useful to not only have the ability to report the checksums that are stored but to recalculate them to determine whether they are valid while reporting them. This is a little different than an fschk. This can be done just for Ext4, but maybe an API call should be added to generalize it.