lofs: multiple readdir problems

[djs55]

On #35 @dsheets comments, " I'd recommend using the dirent index as the read length of directories instead of the 9p stat struct length. This will make progressive listing of directories much faster as stats to all prior entries will not be required (just drop the first offset list items). There is no requirement for directory listing consistency so a snapshot is not required. What if a directory contains more than 1024 entries?"

We could also consider being more strict about what we accept: in particular the 9P spec says that offset must be either 0 (seek to beginning) or the previous offset + the previous data length.