xfrm6: Fix CHECKSUM_COMPLETE after IPv6 header push

xfrm6_transport_finish rebuilds the IPv6 header based on the
original one and pushes it back without fixing skb->csum.
Therefore, CHECKSUM_COMPLETE is no longer valid and the packet
gets dropped.

Fix skb->csum by calling skb_postpush_rcsum.

Note: A valid IPv4 header has checksum 0, unlike IPv6. Thus,
the change is not needed in the sibling xfrm4_transport_finish
function.
Signed-off-by: Yossi Kuperman <yossiku@mellanox.com>
Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/xfrm6_input.c

@@ -34,6 +34,7 @@ EXPORT_SYMBOL(xfrm6_rcv_spi);
 int xfrm6_transport_finish(struct sk_buff *skb, int async)
 {
 	struct xfrm_offload *xo = xfrm_offload(skb);
+	int nhlen = skb->data - skb_network_header(skb);
 
 	skb_network_header(skb)[IP6CB(skb)->nhoff] =
 		XFRM_MODE_SKB_CB(skb)->protocol;
@@ -43,8 +44,9 @@ int xfrm6_transport_finish(struct sk_buff *skb, int async)
 		return 1;
 #endif
 
-	__skb_push(skb, skb->data - skb_network_header(skb));
+	__skb_push(skb, nhlen);
 	ipv6_hdr(skb)->payload_len = htons(skb->len - sizeof(struct ipv6hdr));
+	skb_postpush_rcsum(skb, skb_network_header(skb), nhlen);
 
 	if (xo && (xo->flags & XFRM_GRO)) {
 		skb_mac_header_rebuild(skb);