bridge: correct IPv6 checksum after pull

Bridge multicast snooping of ICMPv6 would incorrectly report a checksum problem
when used with Ethernet devices like sky2 that use CHECKSUM_COMPLETE.
When bytes are removed from skb, the computed checksum needs to be adjusted.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Tested-by: Martin Volf <martin.volf.42@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_multicast.c

@@ -1501,6 +1501,8 @@ static int br_multicast_ipv6_rcv(struct net_bridge *br,
 
 	__skb_pull(skb2, offset);
 	skb_reset_transport_header(skb2);
+	skb_postpull_rcsum(skb2, skb_network_header(skb2),
+			   skb_network_header_len(skb2));
 
 	icmp6_type = icmp6_hdr(skb2)->icmp6_type;