NFS: Simplify arguments of encode_renew()

Clean up: pass just the clientid4 to encode_renew().  This enables it
to be used by callers who might not have an full nfs_client.

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