[Yaffs] Checkin of Todd's readdir change

Harking back to
http://stoneboat.aleph1.co.uk/pipermail/yaffs/2005q4/001645.html

I did npt check this in immediately because I thought a fix along the lines of
that used in yaffs Direct would be "better".

I investigated this further, and even part-wrote the "better" fix. However,
iit turns out that most other file systems use a mechanism in tune with
Todd's so this is what I checked in.

-- CHarles