ipv4: Fix NULL vs error pointer check in inet_blackhole_dev_init()

The inetdev_init() function never returns NULL.  Check for error
pointers instead.

Fixes: 2260059 ("ipv4: give an IPv4 dev to blackhole_netdev")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/aLaQWL9NguWmeM1i@stanley.mountain
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Dan Carpenter

net/ipv4/devinet.c

@@ -340,14 +340,13 @@ static void inetdev_destroy(struct in_device *in_dev)
 
 static int __init inet_blackhole_dev_init(void)
 {
-	int err = 0;
+	struct in_device *in_dev;
 
 	rtnl_lock();
-	if (!inetdev_init(blackhole_netdev))
-		err = -ENOMEM;
+	in_dev = inetdev_init(blackhole_netdev);
 	rtnl_unlock();
 
-	return err;
+	return PTR_ERR_OR_ZERO(in_dev);
 }
 late_initcall(inet_blackhole_dev_init);