netns: ip6mr: dynamically allocates vif6_table

Preliminary work to make IPv6 multicast forwarding netns-aware.

Dynamically allocates interface table vif6_table and moves it to 
struct netns_ipv6, and updates MIF_EXISTS() macro. 

At the moment, vif6_table is only referenced in init_net.

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