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>
diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index 4f00722..edbcccb 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -57,6 +57,8 @@
 
 #ifdef CONFIG_IP_MROUTE
 	struct sock		*mroute_sk;
+	struct vif_device	*vif_table;
+	int			maxvif;
 #endif
 };
 #endif