Add an optional feature to the pNFS server.

Without this patch, the pNFS server distributes the data storage files across
all of the specified DSs.
A tester noted that it would be nice if a system administrator could control
which DSs are used to store the file data for a given exported MDS file system.
This patch adds an optional suffix for each entry in the "-p" option argument
that specifies "store file data for this MDS file system" in this DS.
The patch should only affect sites using the pNFS server (specified via the
"-p" command line option for nfsd.
The interface between the nfsd and the kernel has changed with this patch,
so anyone using the "-p" option needs to rebuild their nfsd from sources
with this patch applied to them.

Discussed with: james.rose@framestore.com