lockd: document use of mon_id argument in SM_MON requests

Clean up: document the argument type that xdr_encode_common() is
marshalling by introducing a new function.  The new function will replace
xdr_encode_common() in just a sec.

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