RPC: populate local address for rendezvous transporter.

When accepting a connection on underlying tcp socket rpc vc
transporter fails to populate local address. This change rectifies
this problem by modifying rendezvous_request() to fill out the xptr
structure member with appropriate information.

Submitted by: Alex Burlyga <alex.burlyga.ietf@gmail.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D4206