rtnetlink: pass link_net to the newlink handler

When IFLA_LINK_NETNSID is used, the netdevice should be built in this link netns
and moved at the end to another netns (pointed by the socket netns or
IFLA_NET_NS_[PID|FD]).

Existing user of the newlink handler will use the netns argument (src_net) to
find a link netdevice or to check some other information into the link netns.
For example, to find a netdevice, two information are required: an ifindex
(usually from IFLA_LINK) and a netns (this link netns).

Note: when using IFLA_LINK_NETNSID and IFLA_NET_NS_[PID|FD], a user may create a
netdevice that stands in netnsX and with its link part in netnsY, by sending a
rtnl message from netnsZ.
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2148,7 +2148,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh)
 		dev->ifindex = ifm->ifi_index;
 
 		if (ops->newlink) {
-			err = ops->newlink(net, dev, tb, data);
+			err = ops->newlink(link_net ? : net, dev, tb, data);
 			/* Drivers should call free_netdev() in ->destructor
 			 * and unregister it on failure after registration
 			 * so that device could be finally freed in rtnl_unlock.