[NETFILTER]: xt_TOS: Change semantic of mask value

This patch changes the behavior of xt_TOS v1 so that the mask value
the user supplies means "zero out these bits" rather than "keep these
bits". This is more easy on the user, as (I would assume) people keep
more bits than zeroing, so, an example:

	Action:     Set bit 0x01.
    	before (&): iptables -j TOS --set-tos 0x01/0xFE
    	after (&~): iptables -j TOS --set-tos 0x01/0x01

This is not too "tragic" with xt_TOS, but where larger fields are used
(e.g. proposed xt_MARK v2), `--set-xmar 0x01/0x01` vs. `--set-xmark
0x01/0xFFFFFFFE` really makes a difference. Other target(!) modules,
such as xt_TPROXY also use &~ rather than &, so let's get to a common
ground.

(Since xt_TOS has not yet left the development tree en direction to
mainline, the semantic can be changed as proposed without breaking
iptables.)
Signed-off-by: NJan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/xt_DSCP.c

@@ -128,7 +128,7 @@ tos_tg(struct sk_buff *skb, const struct net_device *in,
 	u_int8_t orig, nv;
 
 	orig = ipv4_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)))