SUNRPC: Simplify synopsis of rpcb_local_clnt()

Clean up: At one point, rpcb_local_clnt() handled IPv6 loopback
addresses too, but it doesn't any more; only IPv4 loopback is used
now.  Get rid of the @addr and @addrlen arguments to
rpcb_local_clnt().

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