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 the kernel support to do this. It also makes a slight semantic
change to nfsv4_findmirror(), since some uses of it no longer require that
the DS being searched for have a current mirror.
A patch that will be committed in a few minutes will modify the nfsd daemon
to support this feature.
The patch should only affect sites using the pNFS server (specified via the
"-p" command line option for nfsd.

Suggested by: james.rose@framestore.com