netdev: remove potentially harmful checks

Currently we're checking a variable for != NULL after actually
dereferencing it, in netdev_lower_get_next_private*().

It's counter-intuitive at best, and can lead to faulty usage (as it implies
that the variable can be NULL), so fix it by removing the useless checks.

Reported-by: Daniel Borkmann <dborkman@redhat.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: Eric Dumazet <edumazet@google.com>
CC: Nicolas Dichtel <nicolas.dichtel@6wind.com>
CC: Jiri Pirko <jiri@resnulli.us>
CC: stephen hemminger <stephen@networkplumber.org>
CC: Jerry Chu <hkchu@google.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -4589,8 +4589,7 @@ void *netdev_lower_get_next_private(struct net_device *dev,
 	if (&lower->list == &dev->adj_list.lower)
 		return NULL;
 
-	if (iter)
-		*iter = lower->list.next;
+	*iter = lower->list.next;
 
 	return lower->private;
 }
@@ -4618,8 +4617,7 @@ void *netdev_lower_get_next_private_rcu(struct net_device *dev,
 	if (&lower->list == &dev->adj_list.lower)
 		return NULL;
 
-	if (iter)
-		*iter = &lower->list;
+	*iter = &lower->list;
 
 	return lower->private;
 }