genetlink: make multicast groups const, prevent abuse

Register generic netlink multicast groups as an array with
the family and give them contiguous group IDs. Then instead
of passing the global group ID to the various functions that
send messages, pass the ID relative to the family - for most
families that's just 0 because the only have one group.

This avoids the list_head and ID in each group, adding a new
field for the mcast group ID offset to the family.

At the same time, this allows us to prevent abusing groups
again like the quota and dropmon code did, since we can now
check that a family only uses a group it owns.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/wimax/wimax-internal.h

@@ -86,7 +86,6 @@ void wimax_rfkill_rm(struct wimax_dev *);
 
 /* generic netlink */
 extern struct genl_family wimax_gnl_family;
-extern struct genl_multicast_group wimax_gnl_mcg;
 
 /* ops */
 int wimax_gnl_doit_msg_from_user(struct sk_buff *skb, struct genl_info *info);