Fix handling of the hybrid DS case for a pNFS server.

After the addition of the "#mds_path" suffix for a DS specification on the
"-p" nfsd option, it is possible to have a mix of DSs assigned to an MDS
file system and DSs that store files for all DSs. This is what I referred
to as "hybrid" above.
At first, I didn't think this hybrid case would be useful, but I now believe
that some system administrators may fine it useful.
This patch modifies the file storage assignment algorithm so that it
makes the "#mds_path" DSs take priority and the all file systems DSs
are now only used for MDS file systems with no "#mds_path" DS servers.
This only affects the pNFS server for this "hybrid" case.