net: ethtool: use correct device pointer in ethnl_default_dump_one()

net: ethtool: use correct device pointer in ethnl_default_dump_one()

ethnl_default_dump_one() operates on the device provided in its @dev
parameter, not from ctx->req_info->dev.

syzbot reported:

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000197: 0000 [#1] PREEMPT SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000cb8-0x0000000000000cbf]
 RIP: 0010:netdev_need_ops_lock include/linux/netdevice.h:2792 [inline]
 RIP: 0010:netdev_lock_ops include/linux/netdevice.h:2803 [inline]
 RIP: 0010:ethnl_default_dump_one net/ethtool/netlink.c:557 [inline]
 RIP: 0010:ethnl_default_dumpit+0x447/0xd40 net/ethtool/netlink.c:593
Call Trace:
 <TASK>
  genl_dumpit+0x10d/0x1b0 net/netlink/genetlink.c:1027
  netlink_dump+0x64d/0xe10 net/netlink/af_netlink.c:2309
  __netlink_dump_start+0x5a2/0x790 net/netlink/af_netlink.c:2424
  genl_family_rcv_msg_dumpit net/netlink/genetlink.c:1076 [inline]
  genl_family_rcv_msg net/netlink/genetlink.c:1192 [inline]
  genl_rcv_msg+0x894/0xec0 net/netlink/genetlink.c:1210
  netlink_rcv_skb+0x206/0x480 net/netlink/af_netlink.c:2534
  genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
  netlink_unicast_kernel net/netlink/af_netlink.c:1313 [inline]
  netlink_unicast+0x7f6/0x990 net/netlink/af_netlink.c:1339
  netlink_sendmsg+0x8de/0xcb0 net/netlink/af_netlink.c:1883
  sock_sendmsg_nosec net/socket.c:709 [inline]
  __sock_sendmsg+0x221/0x270 net/socket.c:724
  ____sys_sendmsg+0x53a/0x860 net/socket.c:2564
  ___sys_sendmsg net/socket.c:2618 [inline]
  __sys_sendmsg+0x269/0x350 net/socket.c:2650

Fixes: 2bcf4772e45a ("net: ethtool: try to protect all callback with netdev instance lock")
Reported-by: syzbot+3da2442641f0c6a705a2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/67caaf5e.050a0220.15b4b9.007a.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250307083544.1659135-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>