mlxsw: spectrum_router: Correctly handle identical routes

mlxsw: spectrum_router: Correctly handle identical routes

In the device, routes are indexed in a routing table based on the prefix
and its length. This is in contrast to the kernel's FIB where several
FIB aliases can exist with these parameters being identical. In such
cases, the routes will be sorted by table ID (LOCAL first, then MAIN),
TOS and finally priority (metric).

During lookup, these routes will be evaluated in order. In case the
packet's TOS field is non-zero and a FIB alias with a matching TOS is
found, then it's selected. Otherwise, the lookup defaults to the route
with TOS 0 (if it exists). However, if the requested scope is narrower
than the one found, then the lookup continues.

To best reflect the kernel's datapath we should take the above into
account. Given a prefix and its length, the reflected route will always
be the first one in the FIB alias list. However, if the route has a
non-zero TOS then its action will be converted to trap instead of
forward, since we currently don't support TOS-based routing. If this
turns out to be a real issue, we can add support for that using
policy-based switching.

The route's scope can be effectively ignored as any packet being routed
by the device would've been looked-up using the widest scope (UNIVERSE).

To achieve that we need to do two changes. Firstly, we need to create
another struct (FIB node) that will hold the list of FIB entries sharing
the same prefix and length. This struct will be hashed using these two
parameters.

Secondly, we need to change the route reflection to match the above
logic, so that the first FIB entry in the list will be programmed into
the device while the rest will remain in the driver's cache in case of
subsequent changes.

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>