mlxsw: spectrum_router: Implement private fib

Shadow FIB is needed in order to hold additional information for FIB
entries and keep track of used prefixes. That is needed for the LPM tree
construction to be introduced later on in this set.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>