rtnetlink: fix rtnl message size computation for XDP

rtnl_xdp_size() only considers the size of the actual payload attribute,
and misses the space taken by the attribute used for nesting (IFLA_XDP).

Fixes: d1fdd91386 ("rtnl: add option for setting link xdp prog")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Brenden Blanco <bblanco@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -901,7 +901,8 @@ static size_t rtnl_port_size(const struct net_device *dev,
 
 static size_t rtnl_xdp_size(const struct net_device *dev)
 {
-	size_t xdp_size = nla_total_size(1);	/* XDP_ATTACHED */
+	size_t xdp_size = nla_total_size(0) +	/* nest IFLA_XDP */
+			  nla_total_size(1);	/* XDP_ATTACHED */
 
 	if (!dev->netdev_ops->ndo_xdp)
 		return 0;