tunnels: correct conditional build of MPLS and IPv6

Using a combination if #if conditionals and goto labels to unwind
tunnel4_init seems unwieldy. This patch takes a simpler approach of
directly unregistering previously registered protocols when an error
occurs.

This fixes a number of problems with the current implementation
including the potential presence of labels when they are unused
and the potential absence of unregister code when it is needed.

Fixes: 8afe97e5 ("tunnels: support MPLS over IPv4 tunnels")
Signed-off-by: NSimon Horman <simon.horman@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tunnel4.c

@@ -208,24 +208,25 @@ static const struct net_protocol tunnelmpls4_protocol = {
 static int __init tunnel4_init(void)
 {
 	if (inet_add_protocol(&tunnel4_protocol, IPPROTO_IPIP))
-		goto err_ipip;
+		goto err;
 #if IS_ENABLED(CONFIG_IPV6)
-	if (inet_add_protocol(&tunnel64_protocol, IPPROTO_IPV6))
-		goto err_ipv6;
+	if (inet_add_protocol(&tunnel64_protocol, IPPROTO_IPV6)) {
+		inet_del_protocol(&tunnel4_protocol, IPPROTO_IPIP);
+		goto err;
+	}
 #endif
 #if IS_ENABLED(CONFIG_MPLS)
-	if (inet_add_protocol(&tunnelmpls4_protocol, IPPROTO_MPLS))
-		goto err_mpls;
+	if (inet_add_protocol(&tunnelmpls4_protocol, IPPROTO_MPLS)) {
+		inet_del_protocol(&tunnel4_protocol, IPPROTO_IPIP);
+#if IS_ENABLED(CONFIG_IPV6)
+		inet_del_protocol(&tunnel64_protocol, IPPROTO_IPV6);
+#endif
+		goto err;
+	}
 #endif
 	return 0;
 
-#if IS_ENABLED(CONFIG_IPV6)
-err_mpls:
-	inet_del_protocol(&tunnel4_protocol, IPPROTO_IPV6);
-#endif
-err_ipv6:
-	inet_del_protocol(&tunnel4_protocol, IPPROTO_IPIP);
-err_ipip:
+err:
 	pr_err("%s: can't add protocol\n", __func__);
 	return -EAGAIN;
 }