netfilter: xt_TCPOPTSTRIP: signed tcphoff for ipv6_skip_exthdr() retval

if tcphoff remains unsigned, a negative ipv6_skip_exthdr() return value will
go unnoticed,

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/xt_TCPOPTSTRIP.c

@@ -90,7 +90,7 @@ tcpoptstrip_tg6(struct sk_buff *skb, const struct net_device *in,
 		const struct xt_target *target, const void *targinfo)
 {
 	struct ipv6hdr *ipv6h = ipv6_hdr(skb);
-	unsigned int tcphoff;
+	int tcphoff;
 	u_int8_t nexthdr;
 
 	nexthdr = ipv6h->nexthdr;