net: Add per-netns netdev unregistration infra.

When we need to unregister a netdev in a different netns, we will
delegate its unregistration to per-netns work.

There are three types of such cross-netns devices:

  1. Paired devices (e.g., netkit, veth, vxcan)
     -> Unregistering one device also deletes its peer, which
        may reside in another netns.

  2. Tunnel devices (e.g., bareudp, geneve, etc)
     -> Destroying a netns removes devices in another netns if
        their backend sockets reside in the dying netns

  3. Stacked devices (e.g., ipvlan, macvlan, etc)
     -> Removing the lower device also removes multiple upper
        devices, each of which may reside in different namespaces.

In these cases, we will use unregister_netdevice_queue_net() to
queue such potential cross-netns devices for destruction.

Each driver must not call both unregister_netdevice_queue_net()
and unregister_netdevice_queue() for the same device.  See the
subsequent veth/bareudp/ipvlan patches for how they avoid double
queueing.

unregister_netdevice_queue_net() takes net and dev.  If dev resides
in the net, it simply calls unregister_netdevice_queue().

If dev_net(dev) is different from the net, it enqueues the device
to dev_net(dev)->dev_unreg_head and schedules the per-netns work.

When __rtnl_net_unlock() is called from the per-netns work (or another
thread already holding the lock), unregister_netdevice_many_net()
collects the queued devices and calls unregister_netdevice_many()
to perform the actual unregistration.

During netns dismantle, rtnl_net_flush_workqueue() is called at the
end of default_device_exit_batch() to ensure that cross-netns
devices in the other alive netns are unregistered.

Once RTNL is removed, a device could be moved to another netns while
being queued to net->dev_unreg_head.

__dev_change_net_namespace() handles this race by acquiring
net->dev_unreg_lock of both the old and new netns after dev_set_net()
and moving the device between their dev_unreg_head lists.

Since dev_set_net() and unregister_netdevice_queue_net() are
synchronised by netdev_lock(), the device is either queued to the
old netns's dev_unreg_head and then moved, or queued directly to
the new netns.

Note that unregister_netdevice_move_net() does not need to call
rtnl_net_queue_work() because __dev_change_net_namespace() is
(supposed to be) called with rtnl_net_lock().  (Not all callers
hold it yet, but the race does not happen until all callers
are converted and RTNL is removed.)

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260703001009.1572444-7-kuniyu@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>