Add an optional "#<mds-export-path>" field to the elements for the "-p"
option that defines the DSs on the MDS.
This optional field specifies that the MDS should only store files for the
specified MDS exported file system on the DS.
It allows a sysadmin to partition the storage on the DSs, based on exported
MDS file systems.
This patch consists of the kernel changes needed for this to be done.