net: dsa: make user ports return to init_net on netns deletion

As pointed out during review, currently the following set of commands
crashes the kernel:

$ ip netns add ns0
$ ip link set swp0 netns ns0
$ ip netns del ns0
WARNING: CPU: 1 PID: 27 at net/core/dev.c:10884 unregister_netdevice_many+0xaa4/0xaec
Workqueue: netns cleanup_net
pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : unregister_netdevice_many+0xaa4/0xaec
lr : unregister_netdevice_many+0x700/0xaec
Call trace:
 unregister_netdevice_many+0xaa4/0xaec
 default_device_exit_batch+0x294/0x340
 ops_exit_list+0xac/0xc4
 cleanup_net+0x2e4/0x544
 process_one_work+0x4ec/0xb40
---[ end trace 0000000000000000 ]---
unregister_netdevice: waiting for swp0 to become free. Usage count = 2

This is because since DSA user ports, since they started populating
dev->rtnl_link_ops in the blamed commit, gained a different treatment
from default_device_exit_net(), which thinks these interfaces can now be
unregistered.

They can't; so set netns_refund = true to restore the behavior prior to
populating dev->rtnl_link_ops.

Fixes: 95f510d0b7 ("net: dsa: allow the DSA master to be seen and changed through rtnetlink")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220921185428.1767001-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/netlink.c

@@ -59,4 +59,5 @@ struct rtnl_link_ops dsa_link_ops __read_mostly = {
 	.changelink		= dsa_changelink,
 	.get_size		= dsa_get_size,
 	.fill_info		= dsa_fill_info,
+	.netns_refund		= true,
 };