geneve: should not call rt6_lookup() when ipv6 was disabled

When we add a new GENEVE device with IPv6 remote, checking only for
IS_ENABLED(CONFIG_IPV6) is not enough as we may disable IPv6 in the
kernel command line (ipv6.disable=1), and calling rt6_lookup() would
cause a NULL pointer dereference.

v2:
- don't mix declarations and code (reported by Stefano Brivio, Eric Dumazet)
- there's no need to use in6_dev_get() as we only need to check that
  idev exists (reported by David Ahern). This is under RTNL, so we can
  simply use __in6_dev_get() instead (Stefano, Eric).

Reported-by: Jianlin Shi <jishi@redhat.com>
Fixes: c40e89fd35 ("geneve: configure MTU based on a lower device")
Cc: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/geneve.c

@@ -1512,9 +1512,13 @@ static void geneve_link_config(struct net_device *dev,
 	}
 #if IS_ENABLED(CONFIG_IPV6)
 	case AF_INET6: {
-		struct rt6_info *rt = rt6_lookup(geneve->net,
-						 &info->key.u.ipv6.dst, NULL, 0,
-						 NULL, 0);
+		struct rt6_info *rt;
+
+		if (!__in6_dev_get(dev))
+			break;
+
+		rt = rt6_lookup(geneve->net, &info->key.u.ipv6.dst, NULL, 0,
+				NULL, 0);
 
 		if (rt && rt->dst.dev)
 			ldev_mtu = rt->dst.dev->mtu - GENEVE_IPV6_HLEN;