Preliminary work to make IPv4 multicast routing netns-aware. Dynamically allocate IPv4 multicast forwarding cache, mfc_cache_array, and move it to struct netns_ipv4. At the moment, mfc_cache_array is only referenced in init_net. Signed-off-by: Benjamin Thery <benjamin.thery@bull.net> Signed-off-by: David S. Miller <davem@davemloft.net>