Fix forced dismount when a pNFS mount is hung on a DS.

When a "pnfs" NFSv4.1 mount is hung because of an unresponsive DS,
a forced dismount wouldn't work, because the RPC socket for the DS
was not being closed. This patch fixes this.
This will only affect "pnfs" mounts where the pNFS server's DS
is unresponsive (crashed or network partitioned or...).
Found during testing of the pNFS server.

MFC after: 2 weeks