NFS: Add method to detect whether an FSID is still on the server

Introduce a mechanism for probing a server to determine if an FSID
is present or absent.

The on-the-wire compound is different between minor version 0 and 1.
Minor version 0 appends a RENEW operation to identify which client
ID is probing.  Minor version 1 has a SEQUENCE operation in the
compound which effectively carries the same information.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>