rtnetlink: Lookup device in target netns when creating link

When creating link, lookup for existing device in target net namespace
instead of current one.
For example, two links created by:

# ip link add dummy1 type dummy
# ip link add netns ns1 dummy1 type dummy

should have no conflict since they are in different namespaces.

Signed-off-by: Xiao Liang <[e-mail]>
Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/rtnetlink.c

···
 {
 	struct nlattr ** const tb = tbs->tb;
 	struct net *net = sock_net(skb->sk);
+	struct net *device_net;
 	struct net_device *dev;
 	struct ifinfomsg *ifm;
 	bool link_specified;
 
+	/* When creating, lookup for existing device in target net namespace */
+	device_net = (nlh->nlmsg_flags & NLM_F_CREATE) &&
+		     (nlh->nlmsg_flags & NLM_F_EXCL) ?
+		     tgt_net : net;
+
 	ifm = nlmsg_data(nlh);
 	if (ifm->ifi_index > 0) {
 		link_specified = true;
-		dev = __dev_get_by_index(net, ifm->ifi_index);
+		dev = __dev_get_by_index(device_net, ifm->ifi_index);
 	} else if (ifm->ifi_index < 0) {
 		NL_SET_ERR_MSG(extack, "ifindex can't be negative");
 		return -EINVAL;
 	} else if (tb[IFLA_IFNAME] || tb[IFLA_ALT_IFNAME]) {
 		link_specified = true;
-		dev = rtnl_dev_get(net, tb);
+		dev = rtnl_dev_get(device_net, tb);
 	} else {
 		link_specified = false;
 		dev = NULL;