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>