[pnfs] page sync model

[Benny Halevy]

I've updated http://www.bhalevy.com/pnfs/pnfs-page-sync-model.jpg
with the error paths as Dean requested and added the "update layout"
state, preceding write via LD.

To keep it simple, I suggest all error paths go back to the initial
*dirty* state, although some operations (e.g. commit) can be retried
on some error paths.  This way we can keep error handling at one place
where we can decide whether to retry via pNFS - return the layout
we have and get a new one, or fall back to the legacy write path through
the MDS.

Benny