net: bridge: fix sign bug

ipv6_skip_exthdr() can return error code that is below zero.
'offset' is unsigned, so it makes no sense.
ipv6_skip_exthdr() returns 'int' so we can painlessly change type of
offset to int.
Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_multicast.c

@@ -1435,7 +1435,7 @@ static int br_multicast_ipv6_rcv(struct net_bridge *br,
 	struct icmp6hdr *icmp6h;
 	u8 nexthdr;
 	unsigned len;
-	unsigned offset;
+	int offset;
 	int err;
 
 	if (!pskb_may_pull(skb, sizeof(*ip6h)))