selftests: mlxsw: Add test for FIB offload API

The test reuses the common FIB offload tests in order to make sure that
mlxsw correctly implements FIB offload.

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