rpc_generic.c: Fix a rpcbind core dump when rpcinfo is done

Commit c5d671b added netlink support to
server side rpcbind. However it did not add
a case for AF_NETLINK to __rpc_taddr2uaddr_af().
(Reported as PR#289625.)

As such, without this patch the r_addr field of the
netlink rbllist is NULL, which causes a crash in
svc_sendreply() for a Dump query (what rpcinfo
does).

PR: 289625
Reviewed by: glebius
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52651
Fixes: c5d671b711c3 ("libc/rpc: add userland side RPC server over netlink(4)")