FileStore: notice that the ObjectContext disagrees with the on-disk object whenever it's free
We don't check in the OSD that the data we get back when doing a filesystem read of an object matches any reasonable expectations we have based on the ObjectContext. In some cases we are already doing stat() and should check that it agrees with the ObjectContext. In others we can make sure that we can read the right amount of data from an object and that it doesn't turn up short.
This will make us more robust against silent failures of the local FS, malicious users, etc, by letting us detect missing data in more cases and handle it as missing data instead of as data that never existed.