vrf: track associations between VRF devices and tables

Add the data structures and the processing logic to keep track of the
associations between VRF devices and routing tables.
When a VRF is instantiated, it needs to refer to a given routing table.
For each table, we explicitly keep track of the existing VRFs that refer to
the table.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: David S. Miller <davem@davemloft.net>