netns: ipmr: declare reg_vif_num per-namespace

Preliminary work to make IPv4 multicast routing netns-aware.

Declare variable 'reg_vif_num' per-namespace, move into struct netns_ipv4.

At the moment, this variable is only referenced in init_net.

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