netns: ipmr: declare ipmr /proc/net entries per-namespace

Declare IPv4 multicast forwarding /proc/net entries per-namespace:
/proc/net/ip_mr_vif
/proc/net/ip_mr_cache

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