NFS: Enable NFS client to generate CLIENTID strings with IPv6 addresses

We recently added methods to RPC transports that provide string versions of
the remote peer address information.  Convert the NFSv4 SETCLIENTID
procedure to use those methods instead of building the client ID out of
whole cloth.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Aurelien Charbon <aurelien.charbon@ext.bull.net>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>