lockd: introduce new function to encode private argument in SM_MON requests

Clean up: refactor the encoding of the opaque 16-byte private argument in
xdr_encode_mon().  This will be updated later to support IPv6 addresses.

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