Add a command the displays and modifies the pNFS server's extended attribute.

This command allows a sysadmin to display or modify the pnfsd.dsfile extended
attribute used by the pNFS MDS server in various ways.
Its main use is to set a DS's IP address to 0.0.0.0 when that DS has failed,
so that it will not be used for the file when brought back online after
being repaired.