tcp: Mark v6 response packets as CHECKSUM_PARTIAL

Otherwise we only get the checksum right for data-less TCP responses.

Noticed by Herbert Xu.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 5d2e430..1ababbb 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1054,6 +1054,9 @@
 	ipv6_addr_copy(&fl.fl6_dst, &ipv6_hdr(skb)->saddr);
 	ipv6_addr_copy(&fl.fl6_src, &ipv6_hdr(skb)->daddr);
 
+	buff->ip_summed = CHECKSUM_PARTIAL;
+	buff->csum = 0;
+
 	__tcp_v6_send_check(buff, &fl.fl6_src, &fl.fl6_dst);
 
 	fl.proto = IPPROTO_TCP;