macvtap: advertise link netns via netlink

Assign rtnl_link_ops->get_link_net() callback so that IFLA_LINK_NETNSID is
added to rtnetlink messages. This fixes iproute2 which otherwise resolved
the link interface to an interface in the wrong namespace.

Test commands:

  ip netns add nst
  ip link add dummy0 type dummy
  ip link add link macvtap0 link dummy0 type macvtap
  ip link set macvtap0 netns nst
  ip -netns nst link show macvtap0

Before:

  10: macvtap0@gre0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 500
      link/ether 5e:8f:ae:1d:60:50 brd ff:ff:ff:ff:ff:ff

After:

  10: macvtap0@if2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 500
      link/ether 5e:8f:ae:1d:60:50 brd ff:ff:ff:ff:ff:ff link-netnsid 0
Reported-by: Leonardo Mörlein <freifunk@irrelefant.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Link: https://lore.kernel.org/r/20220228003240.1337426-1-sven@narfation.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/macvtap.c

@@ -133,11 +133,17 @@ static void macvtap_setup(struct net_device *dev)
 	dev->tx_queue_len = TUN_READQ_SIZE;
 }
 
+static struct net *macvtap_link_net(const struct net_device *dev)
+{
+	return dev_net(macvlan_dev_real_dev(dev));
+}
+
 static struct rtnl_link_ops macvtap_link_ops __read_mostly = {
 	.kind		= "macvtap",
 	.setup		= macvtap_setup,
 	.newlink	= macvtap_newlink,
 	.dellink	= macvtap_dellink,
+	.get_link_net	= macvtap_link_net,
 	.priv_size      = sizeof(struct macvtap_dev),
 };