netfilter: IPv6: fix DSCP mangle code

The mask indicates the bits one wants to zero out, so it needs to be
inverted before applying to the original TOS field.
Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_DSCP.c

@@ -99,7 +99,7 @@ tos_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 	u_int8_t orig, nv;
 
 	orig = ipv6_get_dsfield(iph);
-	nv   = (orig & info->tos_mask) ^ info->tos_value;
+	nv   = (orig & ~info->tos_mask) ^ info->tos_value;
 
 	if (orig != nv) {
 		if (!skb_make_writable(skb, sizeof(struct iphdr)))