rtnetlink: Try the outer netns attribute in rtnl_get_peer_net().

Xiao Liang reported that the cited commit changed netns handling
in newlink() of netkit, veth, and vxcan.

Before the patch, if we don't find a netns attribute in the peer
device attributes, we tried to find another netns attribute in
the outer netlink attributes by passing it to rtnl_link_get_net().

Let's restore the original behaviour.

Fixes: 4832756 ("rtnetlink: fix double call of rtnl_link_get_net_ifla()")
Reported-by: Xiao Liang <[email protected]>
Closes: https://lore.kernel.org/netdev/CABAhCORBVVU8P6AHcEkENMj+gD2d3ce9t=A_o48E0yOQp8_wUQ@mail.gmail.com/#t
Signed-off-by: Kuniyuki Iwashima <[email protected]>
Tested-by: Xiao Liang <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: q2ven

net/core/rtnetlink.c

@@ -3819,14 +3819,15 @@ static int rtnl_newlink_create(struct sk_buff *skb, struct ifinfomsg *ifm,
 }
 
 static struct net *rtnl_get_peer_net(const struct rtnl_link_ops *ops,
+				     struct nlattr *tbp[],
 				     struct nlattr *data[],
 				     struct netlink_ext_ack *extack)
 {
 	struct nlattr *tb[IFLA_MAX + 1];
 	int err;
 
 	if (!data || !data[ops->peer_type])
-		return NULL;
+		return rtnl_link_get_net_ifla(tbp);
 
 	err = rtnl_nla_parse_ifinfomsg(tb, data[ops->peer_type], extack);
 	if (err < 0)
@@ -3971,7 +3972,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 		}
 
 		if (ops->peer_type) {
-			peer_net = rtnl_get_peer_net(ops, data, extack);
+			peer_net = rtnl_get_peer_net(ops, tb, data, extack);
 			if (IS_ERR(peer_net)) {
 				ret = PTR_ERR(peer_net);
 				goto put_ops;