mpls: fix sending of local encapped packets

Locally generated IPv4 and (probably) IPv6 packets are dropped because
skb->protocol isn't set. We could write wrappers to lwtunnel_output
for IPv4 and IPv6 that set the protocol accordingly and then call
lwtunnel_output, but mpls_output relies on the AF-specific type of dst
anyway to get the via address.

Therefore, make use of dst->dst_ops->family in mpls_output to
determine the type of nexthop and thus protocol of the packet instead
of checking skb->protocol.

Fixes: 61adedf3e3f1 ("route: move lwtunnel state to dst_entry")
Reported-by: Sam Russell <sam.h.russell@gmail.com>
Signed-off-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mpls/mpls_iptunnel.c

@@ -54,10 +54,10 @@ int mpls_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 	unsigned int ttl;
 
 	/* Obtain the ttl */
-	if (skb->protocol == htons(ETH_P_IP)) {
+	if (dst->ops->family == AF_INET) {
 		ttl = ip_hdr(skb)->ttl;
 		rt = (struct rtable *)dst;
-	} else if (skb->protocol == htons(ETH_P_IPV6)) {
+	} else if (dst->ops->family == AF_INET6) {
 		ttl = ipv6_hdr(skb)->hop_limit;
 		rt6 = (struct rt6_info *)dst;
 	} else {