Fix some krpc leaks for the NFSv4.1/pNFS client.

The NFSv4.1/pNFS client wasn't doing a newnfs_disconnect() call for the
connection to the Data Server (DS) under some circumstances. The main
effect of this was a leak of malloc'd structures in the krpc. This patch
adds the newnfs_disconnect() calls to fix this.
Detected during recent testing against the pNFS server under development.

MFC after: 2 weeks