[NETLIKN]: Allow removing multicast groups.

Allow kicking listeners out of a multicast group when necessary
(for example if that group is going to be removed.)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Patrick McHardy <kaber@trash.net>
Acked-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>