[NETFILTER]: Fix HW checksum handling in TCPMSS target

Most importantly, remove bogus BUG() in receive path.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ipt_TCPMSS.c

@@ -61,6 +61,10 @@ ipt_tcpmss_target(struct sk_buff **pskb,
 	if (!skb_ip_make_writable(pskb, (*pskb)->len))
 		return NF_DROP;
 
+	if ((*pskb)->ip_summed == CHECKSUM_HW &&
+	    skb_checksum_help(*pskb, out == NULL))
+		return NF_DROP;
+
 	iph = (*pskb)->nh.iph;
 	tcplen = (*pskb)->len - iph->ihl*4;
 
@@ -186,9 +190,6 @@ ipt_tcpmss_target(struct sk_buff **pskb,
 	       newmss);
 
  retmodified:
-	/* We never hw checksum SYN packets.  */
-	BUG_ON((*pskb)->ip_summed == CHECKSUM_HW);
-
 	(*pskb)->nfcache |= NFC_UNKNOWN | NFC_ALTERED;
 	return IPT_CONTINUE;
 }