[patch] Multicast: Keep membership and filters in sync

[Stef Walter]

When removing multicast membership from a socket (ie:
IP_DROP_MEMBERSHIP) that has multiple multicast memberships, the
internal list of memberships and filters are not kept in sync.

This results in dropped packets that are not delivered to the socket
that has the multicast membership. This was experienced with OSPF
(running quagga).

Besides the obvious non-functional multicast, the following command is
another way to see an indication of the problem:

> # netstat -s -p ip | grep multicast
> 	7 packets received for unknown multicast group

Patch attached which fixes the problem.

Cheers,

Stef
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