sit: allow redirecting ip6ip, ipip and mplsip packets to eth devices

Even though sit transports L3 data (IPv6, IPv4 or MPLS) packets, it
needs to reset the mac_header pointer, so that other parts of the stack
don't mistakenly access the outer header after the packet has been
decapsulated. There are two rx handlers to modify: ipip6_rcv() for the
ip6ip mode and sit_tunnel_rcv() which is used to re-implement the ipip
and mplsip modes of ipip.ko.

This allows to push an Ethernet header to sit packets and redirect
them to an Ethernet device:

  $ tc filter add dev sit0 ingress matchall          \
      action vlan push_eth dst_mac 00:00:5e:00:53:01 \
                           src_mac 00:00:5e:00:53:00 \
      action mirred egress redirect dev eth0

Without this patch, push_eth refuses to add an ethernet header because
the skb appears to already have a MAC header.
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -710,6 +710,8 @@ static int ipip6_rcv(struct sk_buff *skb)
 		 * old iph is no longer valid
 		 */
 		iph = (const struct iphdr *)skb_mac_header(skb);
+		skb_reset_mac_header(skb);
+
 		err = IP_ECN_decapsulate(iph, skb);
 		if (unlikely(err)) {
 			if (log_ecn_error)
@@ -780,6 +782,8 @@ static int sit_tunnel_rcv(struct sk_buff *skb, u8 ipproto)
 			tpi = &ipip_tpi;
 		if (iptunnel_pull_header(skb, 0, tpi->proto, false))
 			goto drop;
+		skb_reset_mac_header(skb);
+
 		return ip_tunnel_rcv(tunnel, skb, tpi, NULL, log_ecn_error);
 	}