SUNRPC: Teach rpc_pipe.c to use new rpc_peeraddr() API

Hide the details of how the RPC client stores remote peer addresses from
the RPC pipefs implementation.

Test plan:
Connectathon with Kerberos 5 authentication.

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