netns: ipmr: dynamically allocate vif_table

Preliminary work to make IPv6 multicast routing netns-aware.

Dynamically allocate interface table vif_table and move it to
struct netns_ipv4, and update MIF_EXISTS() macro.

At the moment, vif_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>