[pnfs] stripe unit and server wsize/rsize

[Marc Eshel]

I noticed a problem when the rsize that is set at mount time is bigger 
that the stripe unit. The read size that is used on the first read (before 
getting the layout) is based on the rsize set at mount time and the code 
is not able to do IO that spans more than one DS. We can either fix the 
client code to break the read to multiple DSs or make sure that the server 
limits wsize/rsize to the max stripe unit.
Marc.