bridge: fix a possible use after free

br_multicast_ipv6_rcv() can call pskb_trim_rcsum() and therefore skb
head can be reallocated.

Cache icmp6_type field instead of dereferencing twice the struct
icmp6hdr pointer.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_multicast.c

@@ -1456,7 +1456,7 @@ static int br_multicast_ipv6_rcv(struct net_bridge *br,
 {
 	struct sk_buff *skb2;
 	const struct ipv6hdr *ip6h;
-	struct icmp6hdr *icmp6h;
+	u8 icmp6_type;
 	u8 nexthdr;
 	unsigned len;
 	int offset;
@@ -1502,9 +1502,9 @@ static int br_multicast_ipv6_rcv(struct net_bridge *br,
 	__skb_pull(skb2, offset);
 	skb_reset_transport_header(skb2);
 
-	icmp6h = icmp6_hdr(skb2);
+	icmp6_type = icmp6_hdr(skb2)->icmp6_type;
 
-	switch (icmp6h->icmp6_type) {
+	switch (icmp6_type) {
 	case ICMPV6_MGM_QUERY:
 	case ICMPV6_MGM_REPORT:
 	case ICMPV6_MGM_REDUCTION:
@@ -1544,7 +1544,7 @@ static int br_multicast_ipv6_rcv(struct net_bridge *br,
 
 	BR_INPUT_SKB_CB(skb)->igmp = 1;
 
-	switch (icmp6h->icmp6_type) {
+	switch (icmp6_type) {
 	case ICMPV6_MGM_REPORT:
 	    {
 		struct mld_msg *mld;