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

(cherry picked from commit 70730cd21c9f2b0a80ff07e4491c0fe30f2d87c5)