net: prevent a NULL deref in rtnl_create_link()

net: prevent a NULL deref in rtnl_create_link()

At the time rtnl_create_link() is running, dev->netdev_ops is NULL,
we must not use netdev_lock_ops() or risk a NULL deref if
CONFIG_NET_SHAPER is defined.

Use netif_set_group() instead of dev_set_group().

 RIP: 0010:netdev_need_ops_lock include/net/netdev_lock.h:33 [inline]
 RIP: 0010:netdev_lock_ops include/net/netdev_lock.h:41 [inline]
 RIP: 0010:dev_set_group+0xc0/0x230 net/core/dev_api.c:82
Call Trace:
 <TASK>
  rtnl_create_link+0x748/0xd10 net/core/rtnetlink.c:3674
  rtnl_newlink_create+0x25c/0xb00 net/core/rtnetlink.c:3813
  __rtnl_newlink net/core/rtnetlink.c:3940 [inline]
  rtnl_newlink+0x16d6/0x1c70 net/core/rtnetlink.c:4055
  rtnetlink_rcv_msg+0x7cf/0xb70 net/core/rtnetlink.c:6944
  netlink_rcv_skb+0x208/0x470 net/netlink/af_netlink.c:2534
  netlink_unicast_kernel net/netlink/af_netlink.c:1313 [inline]
  netlink_unicast+0x75b/0x8d0 net/netlink/af_netlink.c:1339
  netlink_sendmsg+0x805/0xb30 net/netlink/af_netlink.c:1883
  sock_sendmsg_nosec net/socket.c:712 [inline]

Reported-by: syzbot+9fc858ba0312b42b577e@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6840265f.a00a0220.d4325.0009.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 7e4d784f5810 ("net: hold netdev instance lock during rtnetlink operations")
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250604105815.1516973-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>