gtp: gtp_check_src_ms_ipv4() always return success

gtp_check_src_ms_ipv4() did not find the PDP context matching with the
UE IP address because the memory location is not right, but the result
is inverted by the Boolean "not" operator.  So whatever is the PDP
context, any call to this function is successful.
Signed-off-by: NLionel Gauthier <Lionel.Gauthier@eurecom.fr>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/gtp.c

@@ -158,9 +158,9 @@ static bool gtp_check_src_ms_ipv4(struct sk_buff *skb, struct pdp_ctx *pctx,
 	if (!pskb_may_pull(skb, hdrlen + sizeof(struct iphdr)))
 		return false;
 
-	iph = (struct iphdr *)(skb->data + hdrlen + sizeof(struct iphdr));
+	iph = (struct iphdr *)(skb->data + hdrlen);
 
-	return iph->saddr != pctx->ms_addr_ip4.s_addr;
+	return iph->saddr == pctx->ms_addr_ip4.s_addr;
 }
 
 /* Check if the inner IP source address in this packet is assigned to any