bare_root_metadata_v3: don't error if MDDiskUsage looks bad
bare_root_metadata_v3: don't error if MDDiskUsage looks bad

As long as the MDRefBytes fields are the same, it probably means that
an old client just copied the value as an unknown field from an MD
written by a new client.

keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client