[GENETLINK]: Fix adjustment of number of multicast groups

The current calculation of the maximum number of genetlink
multicast groups seems odd, fix it.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/genetlink.c

@@ -184,7 +184,7 @@ int genl_register_mc_group(struct genl_family *family,
 	}
 
 	err = netlink_change_ngroups(genl_sock,
-				     sizeof(unsigned long) * NETLINK_GENERIC);
+				     mc_groups_longs * BITS_PER_LONG);
 	if (err)
 		goto out;