netns: ip6mr: enable namespace support in ipv6 multicast forwarding code

This last patch makes the appropriate changes to use and propagate the
network namespace where needed in IPv6 multicast forwarding code.

This consists mainly in replacing all the remaining init_net occurences
with current netns pointer retrieved from sockets, net devices or 
mfc6_caches depending on the routines' contexts.

Some routines receive a new 'struct net' parameter to propagate the current
netns:
* ip6mr_get_route
* ip6mr_cache_report
* ip6mr_cache_find
* ip6mr_cache_unresolved
* mif6_add/mif6_delete
* ip6mr_mfc_add/ip6mr_mfc_delete
* ip6mr_reg_vif

All the IPv6 multicast forwarding variables moved to struct netns_ipv6 by
the previous patches are now referenced in the correct namespace.

Changelog:
==========
* Take into account the net associated to mfc6_cache when matching entries in
  mfc_unres_queue list.
* Call mroute_clean_tables() in ip6mr_net_exit() to free memory allocated
  per-namespace.
* Call dev_net_set() in ip6mr_reg_vif() to initialize dev->nd_net 
  correctly.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/mroute6.h

@@ -245,7 +245,8 @@ void mfc6_net_set(struct mfc6_cache *mfc, struct net *net)
 
 #ifdef __KERNEL__
 struct rtmsg;
-extern int ip6mr_get_route(struct sk_buff *skb, struct rtmsg *rtm, int nowait);
+extern int ip6mr_get_route(struct net *net, struct sk_buff *skb,
+			   struct rtmsg *rtm, int nowait);
 
 #ifdef CONFIG_IPV6_MROUTE
 static inline struct sock *mroute6_socket(struct net *net)