mlxsw: spectrum_router: Add support for route append

When a new route is appended, it's placed after existing routes sharing
the same parameters (prefix, length, table ID, TOS and priority).

While the device supports only one route with the same prefix and length
in a single table, it's important to correctly place the appended route
in the driver's cache, as when a route is deleted the next one is
programmed into the device.

Following the reception of an ENTRY_APPEND notification, resolve the
FIB node corresponding to the prefix and length and correctly place the
new entry in its entry list.

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>