/*
* IPv6 multicast forwarding function. This function assumes that the packet
* pointed to by "ip6" has arrived on (or is about to be sent to) the interface
* pointed to by "ifp", and the packet is to be relayed to other networks
* that have members of the packet's destination IPv6 multicast group.
*
* The packet is returned unscathed to the caller, unless it is
* erroneous, in which case a non-zero return value tells the caller to
* discard it.
*/