[IPV6] MLDv2: fix change records when transitioning to/from inactive

[IPV6] MLDv2: fix change records when transitioning to/from inactive

The following patch fixes these problems in MLDv2:

1) Add/remove "delete" records for sending change reports when
        addition of a filter results in that filter transitioning to/from
        inactive. [same as recent IPv4 IGMPv3 fix]
2) Remove 2 redundant "group_type" checks (can't be IPV6_ADDR_ANY
        within that loop, so checks are always true)
3) change an is_in() "return 0" to "return type == MLD2_MODE_IS_INCLUDE".
        It should always be "0" to get here, but it improves code locality
        to not assume it, and if some race allowed otherwise, doing
        the check would return the correct result.

Signed-off-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>