ethtool: fix null-ptr-deref on ref tracker

dev can be a NULL here, not all requests set require_dev.

Fixes: e4b89540 ("netlink: add net device refcount tracker to struct ethnl_req_info")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ethtool/netlink.c

@@ -141,8 +141,10 @@ int ethnl_parse_header_dev_get(struct ethnl_req_info *req_info,
 		return -EINVAL;
 	}
 
+	if (dev) {
 		req_info->dev = dev;
 		netdev_tracker_alloc(dev, &req_info->dev_tracker, GFP_KERNEL);
+	}
 	req_info->flags = flags;
 	return 0;
 }