ipv6: Fix return value of ipv6_mc_may_pull() for malformed packets

Commit ba5ea61462 ("bridge: simplify ip_mc_check_igmp() and
ipv6_mc_check_mld() calls") replaces direct calls to pskb_may_pull()
in br_ipv6_multicast_mld2_report() with calls to ipv6_mc_may_pull(),
that returns -EINVAL on buffers too short to be valid IPv6 packets,
while maintaining the previous handling of the return code.

This leads to the direct opposite of the intended effect: if the
packet is malformed, -EINVAL evaluates as true, and we'll happily
proceed with the processing.

Return 0 if the packet is too short, in the same way as this was
fixed for IPv4 by commit 083b78a9ed ("ip: fix ip_mc_may_pull()
return value").

I don't have a reproducer for this, unlike the one referred to by
the IPv4 commit, but this is clearly broken.

Fixes: ba5ea61462 ("bridge: simplify ip_mc_check_igmp() and ipv6_mc_check_mld() calls")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Acked-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/addrconf.h

@@ -206,7 +206,7 @@ static inline int ipv6_mc_may_pull(struct sk_buff *skb,
 				   unsigned int len)
 {
 	if (skb_transport_offset(skb) + ipv6_transport_len(skb) < len)
-		return -EINVAL;
+		return 0;
 
 	return pskb_may_pull(skb, len);
 }