IGMP snooping: set mrouters_only flag for IPv6 traffic properly

IGMP snooping: set mrouters_only flag for IPv6 traffic properly

Upon reception of a MGM report packet the kernel sets the mrouters_only flag
in a skb that is a clone of the original skb, which means that the bridge
loses track of MGM packets (cb buffers are tied to a specific skb and not
shared) and it ends up forwading join requests to the bridge interface.

This can cause unexpected membership timeouts and intermitent/permanent loss
of connectivity as described in RFC 4541 [2.1.1. IGMP Forwarding Rules]:

    A snooping switch should forward IGMP Membership Reports only to
    those ports where multicast routers are attached.
    [...]
    Sending membership reports to other hosts can result, for IGMPv1
    and IGMPv2, in unintentionally preventing a host from joining a
    specific multicast group.

Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>