mlxsw: spectrum_router: Prevent incorrect replacement of local table routes

The driver uses the same table to represent both the main and local
routing tables. Prevent routes in the main table from replacing routes
in the local table to reflect the fact that the local table is consulted
first during lookup.

Fixes: b6a1d871 ("mlxsw: spectrum_router: Start using new IPv4 route notifications")
Fixes: dacad7b3 ("mlxsw: spectrum_router: Start using new IPv6 route notifications")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>