ila: Perform only one translation in forwarding path

When setting up ILA in a router we noticed that the the encapsulation
is invoked twice: once in the route input path and again upon route
output. To resolve this we add a flag set_csum_neutral for the
ila_update_ipv6_locator. If this flag is set and the checksum
neutral bit is also set we assume that checksum-neutral translation
has already been performed and take no further action. The
flag is set only in ila_output path. The flag is not set for ila_input and
ila_xlat.

Tested:

Used 3 netns to set to emulate a router and two hosts. The router
translates SIR addresses between the two destinations in other two netns.
Verified ping and netperf are functional.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/ila/ila_lwt.c b/net/ipv6/ila/ila_lwt.c
index 1dfb641..c8314c6 100644
--- a/net/ipv6/ila/ila_lwt.c
+++ b/net/ipv6/ila/ila_lwt.c
@@ -26,7 +26,7 @@
 	if (skb->protocol != htons(ETH_P_IPV6))
 		goto drop;
 
-	ila_update_ipv6_locator(skb, ila_params_lwtunnel(dst->lwtstate));
+	ila_update_ipv6_locator(skb, ila_params_lwtunnel(dst->lwtstate), true);
 
 	return dst->lwtstate->orig_output(net, sk, skb);
 
@@ -42,7 +42,7 @@
 	if (skb->protocol != htons(ETH_P_IPV6))
 		goto drop;
 
-	ila_update_ipv6_locator(skb, ila_params_lwtunnel(dst->lwtstate));
+	ila_update_ipv6_locator(skb, ila_params_lwtunnel(dst->lwtstate), false);
 
 	return dst->lwtstate->orig_input(skb);