ipmr: Add route table ID to netlink cache reports

Insert the multicast route table ID as a Netlink attribute to Netlink
cache report notifications.

When multiple route tables are in use it is necessary to have a way to
determine which route table a given cache report belongs to when
receiving the cache report.

Signed-off-by: Paul Davey <paul.davey@alliedtelesis.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>