When we use netlink to monitor queue information for udp socket,
idiag_rqueue and idiag_wqueue of inet_diag_msg are returned with 0.

Keep consistent with netstat, just return back allocated rmem/wmem size.
Signed-off-by: NShan Wei <davidshan@tencent.com>
Acked-by: NPavel Emelyanov <xemul@parallels.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>