rtnetlink: fix the wrong minimal dump size getting from rtnl_calcit()

For RT netlink, calcit() function should return the minimal size for
netlink dump message. This will make sure that dump message for every
network device can be stored.

Currently, rtnl_calcit() function doesn't account the size of header of
netlink message, this patch will fix it.
Signed-off-by: NZhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2737,7 +2737,7 @@ static u16 rtnl_calcit(struct sk_buff *skb, struct nlmsghdr *nlh)
 						           ext_filter_mask));
 	}
 
-	return min_ifinfo_dump_size;
+	return nlmsg_total_size(min_ifinfo_dump_size);
 }
 
 static int rtnl_dump_all(struct sk_buff *skb, struct netlink_callback *cb)