mlxsw: spectrum_router: Store routes in a more generic way

Up until now, the only FIB entries that were associated with a nexthop
group were routes to remote networks where all the nexthop devices had a
valid router interface (RIF). This is in contrast to the FIB code,
where all the routes are associated with a FIB info. The same design
choice needs to be applied to the driver's cache.

Based on the NH_{ADD,DEL} events which will be added later in the
patchset, we need to be able to change the action (forward / trap)
associated with all the routes using the nexthop group. However, if we
can't link between the nexthop and the routes using it, then the above
is impossible.

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