net: bridge: mcast: use the proper multicast context when dumping router ports

When we are dumping the router ports of a vlan mcast context we need to
use the bridge/vlan and port/vlan's multicast contexts to check if
IPv4/IPv6 router port is present and later to dump the vlan id.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>