net: hold instance lock during NETDEV_CHANGE

net: hold instance lock during NETDEV_CHANGE

Cosmin reports an issue with ipv6_add_dev being called from
NETDEV_CHANGE notifier:

[ 3455.008776]  ? ipv6_add_dev+0x370/0x620
[ 3455.010097]  ipv6_find_idev+0x96/0xe0
[ 3455.010725]  addrconf_add_dev+0x1e/0xa0
[ 3455.011382]  addrconf_init_auto_addrs+0xb0/0x720
[ 3455.013537]  addrconf_notify+0x35f/0x8d0
[ 3455.014214]  notifier_call_chain+0x38/0xf0
[ 3455.014903]  netdev_state_change+0x65/0x90
[ 3455.015586]  linkwatch_do_dev+0x5a/0x70
[ 3455.016238]  rtnl_getlink+0x241/0x3e0
[ 3455.019046]  rtnetlink_rcv_msg+0x177/0x5e0

Similarly, linkwatch might get to ipv6_add_dev without ops lock:
[ 3456.656261]  ? ipv6_add_dev+0x370/0x620
[ 3456.660039]  ipv6_find_idev+0x96/0xe0
[ 3456.660445]  addrconf_add_dev+0x1e/0xa0
[ 3456.660861]  addrconf_init_auto_addrs+0xb0/0x720
[ 3456.661803]  addrconf_notify+0x35f/0x8d0
[ 3456.662236]  notifier_call_chain+0x38/0xf0
[ 3456.662676]  netdev_state_change+0x65/0x90
[ 3456.663112]  linkwatch_do_dev+0x5a/0x70
[ 3456.663529]  __linkwatch_run_queue+0xeb/0x200
[ 3456.663990]  linkwatch_event+0x21/0x30
[ 3456.664399]  process_one_work+0x211/0x610
[ 3456.664828]  worker_thread+0x1cc/0x380
[ 3456.665691]  kthread+0xf4/0x210

Reclassify NETDEV_CHANGE as a notifier that consistently runs under the
instance lock.

Link: https://lore.kernel.org/netdev/aac073de8beec3e531c86c101b274d434741c28e.camel@nvidia.com/
Reported-by: Cosmin Ratiu <cratiu@nvidia.com>
Tested-by: Cosmin Ratiu <cratiu@nvidia.com>
Fixes: ad7c7b2172c3 ("net: hold netdev instance lock during sysfs operations")
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250404161122.3907628-1-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>