netns: ipmr: allocate mroute_socket per-namespace.

Preliminary work to make IPv4 multicast routing netns-aware.

Make IPv4 multicast routing mroute_socket per-namespace,
moves it into struct netns_ipv4.

At the moment, mroute_socket is only referenced in init_net.

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