tipc: simplify signature of tipc_nametbl_lookup_group()

We reduce the signature of tipc_nametbl_lookup_group() by using a
struct tipc_uaddr pointer. This entails a couple of minor changes in the
functions tipc_send_group_mcast/anycast/unicast/bcast() in socket.c

Signed-off-by: Jon Maloy <jmaloy@redhat.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Acked-by: Hoang Le <hoang.h.le@dektech.com.au>
Acked-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Acked-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>